GB2317979A - Synchronisation in a wellbore communication system - Google Patents

Abstract

To synchronise the clock generators at each end of an acoustic wellbore communication system, a first synch signal is sent in a first direction and in response a second synch signal is returned, allowing fine correction. After completion of lowering, a downhole acoustic transducer DAT, begins to send a series of chirp signals through the borehole transmission medium e.g. the drilling fluid. Between chirps the DAT listens for tone signals from the surface acoustic transducer SAT. When the SAT receives the chirps, it synchronises its clock to the incoming signal and characterises the channel by analysing subsequent chirps, as in GB2281424 A. Having identified a utilisable frequency band, the SAT transmits a pair of tone signals to the DAT which uses the received signals to refine its synchronisation. Data interchange then can begin.

Description

2317979 METHOD FOR COMMUNICATING DATA IN A WELLBORE

1 Field of the Invention:

7ne present invention relates to..

(a) a transducer which may be utilized to transmit and receive data in a veilbere', (b) a communication system for improving the communication of data in a wellbore; (c) one application of the transducer in a measurerrient-while-drilling system; and (4) one a.r,,ciicaticn of the transducer and communication system to detect cas influx in a weilbore.

2. Background of the InventigLO:

One of the more difficult problems associated with any borehole 'is to communicate inteiligenc-- between one or more locations down a borehole and the surface, or between downhole locations themselves. For example, communication is desired by the oil industry 'to retrieve, at the surface, data generated downhole during drilling operations, incuding during quiescent periods interspersing actual drilling procedures or whiie tripping; during completion operations such as perforating, iracturing, and drill stem or well testing. and during production operations such as, eser-vor ovaluation testing, pressure and temperature monitoring. Cornmiinication is also desired in such industry to transmit intelligence from the surface to downhole Lools or instruments to effect, control or modify operations or parameters.

Accurate and reliable downhoile communication Is particulariv important when d-ata (intellicence) Is 'c be communicaled. This inteffigence often is in the florm cl' an encoded dicital sicnal.

One approach has been widely considered for bcrehoie communicatIon is to Lse a direct wIre connection bet-ween the surface and the downhole locaticn(s). Communication trien can be via electrical signal through the wire. Whille much efloeL has been expended toward ',,vireline' communication, this a.c.croach has not been adopted commercially because it has been found to be quite cosily and unrellabile. For example, one difficulty with this approach is that since the wire is often laid vii numerous lengths of a drill stem or production tubing, is not unusual for there to be a break or a poor wire connection which arises at the time the wire assembly is first in stalled. While it has been Proposed (see U. S. Patent No. 4,215,426) to avoid the problems associated with direct electriwi coupling of drill stems by prcvl'ding inductive coupling for the communication link at such location, induc,Live coupling has as a problem, among others, major signal loss at every coupling. It also relies on installation of special and complex driiistr':nc arrangements.

Another borehole ccm, munication technique that has been exPlored is 'he 'transmission of acoustic waves. Such physicd waves need a transmission medium that wiii propagate the same. It will be recognized 1 1 1 1 1 that matters such as var'aticns'n eaFth strata, dens ty make-up, etc., render the earth cornpetely ina,[.cro.criate for an acoustic communication transm issicn m edium. Because of these known Problems, those in the art generally have confined themseives to exploring acoustic communication, through, borehoie related m edia.

Much e,c;-, has been expended toward developing an approPriate acoustic communication system in which the borehole drill stem or production tubing itseil acTs as the transmission medium. A m ajor problem, associated with such arrancernents is caused by the fact that the configurations of drill stems or.--rcduci;lcn tubing generally vary signiiwntly lengthwise. These variations typicaily are different in each hole. Moreover, a configuration in a particular borehole may vary over time bewuse, for example, of the addition of tubing and tools to the string. The result Is that ',here is no general usage system relying on drill stem or production tubing 'ss transmi ion that has cained meaningful market acceptance.

Efforts have also been made to utilize liquid within a berehole as the acoustic transmission medium. At first blush, one would think that use of a liquid as the transmission medium in a borehcie would be relatvely simiple aPproach, in view of theMide usage and significant developments that have been made for communication and sonar systems relying on acoustic transmission within the ocean.

Acoustic transmission via a liquid within a borehole is considerabty different than acoustic transmission within an open ocean because of the problems associated with the boundaries between the liquid and ds confining structures in a borehole. Criteria relating to these problems are of paramount importance. However, because of the attracveness of the cf acoustic transmissien In a liquid independent of movement thereof, a system was proposed in U.S. Patent No. 3,964,556 utilizing pressure changes 'in a non-r-ncvIng liquid to communicate. Such system has not been found practical, however, since It is not a self-contained systern and some movement of the liquid has been found necessary to it oressure chances.

Lrans, m, In licht of "7e above, m eaning;iui communication cj intelligence via bcrehole liquids has been 'lml+Led to syster-ns which rely on filow of the liqu!c to carry on acoustic r-,,cdulation from a transmission point to a receiver. This approac,n is a ene rally ref erred,c in 'he art, as MWID (measure wrilie drilling). Developments relating to It have been limited to c-.r,nr-,unication during the drilling phase In the ilile of a borehoie, principally since It is only during drilling that one can be assured of fluid which wn be modulated flowing between the drilling location and the surface. Most MWID systems are also constrained because of the drilling operation itself. For examPle, it is not unusual that IL,- ,e drilling operation must be stopped during communication to avoid the noise associated with such drilling. Moreover, communication during tripping:s impossible.

In spite of the problems with MWID communication, much research has been done on the same in view of the desirability of good bc, ehole communication- 7he result has been an extensive number of Patents relating to MWD, many of which are directed to proposed solutions to the various problems LIat have been encountered. U.S. Patent No. 4,21, 5,426 describes an arrancement in which power (rather than communication) is transmitted downhoie through fiuid modulation akin to MWD communication, a per-tion of which power is drained off at various icc-,2,ions downhole to power re-tp-eaters In a vireine communication 1rans mission systern.

-Fe development cj com, munication using acoustic waves pr--,ca(.-a'inc throuch non-flowing flulds in a bcrehole has been impeded by lack cl' a suitable ransducer. To be practicai for a bcrehole application, a 1,jr-zr-.sdLcer has to in a pressure barrel with an cuter diameter of no r-nore ',,'-,.an 1.25 incilles, operae at temperatures up to 15,01)C and Pressures uP to l CCO bar, and sur,.,i',e the working environment cl' handling and runninc In a weil. Such a z,,aj-,.sducer would also have to take into consideration the slenificant ditferences between communication in a nor,ccns,rained fluid environment, such as Lhe ocean, and a coCined fluid arrangement, such as in a borehole.

The development of reliable communication using accustic waves propagating through non-owine flulds in a bcrehole has been impeded by Lhe fact that the borehole environment Is extremely ncisy. Moreover, to be practical, an accustic communicaiicn system using non-floyAng liquid is require'd 'o be highly ada,o',,'ve ',c ariaicns In the borehole channel and Must Provide robust and reliable throughput of data in sphe of such variations.

SUMMARY OF THE INVENTION

THE TRANSDUCER, The present invention relates to a practical berehole acoustic communication transducer. It Is capable of generating, or responding to, accuszic waves in a viscous liquid confined in a berehole. Its design takes into consideration the waveguide nature of a berehole. it has been found that, to be practicai, a borehole awustic transducer has to generate, or respond to, acoustic waves at frequencies below one kilohertz with bandwidths of tens of Hertz, efficiently in various liquids. It has to be able to do so while Providing high displacement and having a lower mechanical impedance than conventional open ocean devices. The transducer of the invention meets 'these criteria as we!l as the size and operating criteria mentioned above.

The transducer c11 the invention has many features that contribute to Its capaoility. It is similar to a moving coil loudspeaker in that movement of an electric winding relative to magnetic flux in the gap of a macnetic circuit is used to convert between electric power and mechanical motion. It uses the same interaction for transmting and receiving. A dominant feature ofthe transducer of the invention is that a plurality of gaps are used with a corresponding number (and placement) of electrical windings. This facilitates developing, with such a small diameter arrangement, the forces and displacements found to be necessary to transduce the low frequency waves required for adequate transmission through non-flowing viscous fluid confined in a borehole. Moreover, a resonator may be inc!uded as Part of the transducer desired to provide a compliant backicad.

The invention Includes several arrangements responsible for assuring that there is good berehole transmission of acoustic waves. For one, a transition section is included 'to provide acoustic impedance matching in the borehole liquid between sections of the borehoie having significantly different cross- sectio na areas suc, as between the section of the borehole having the transducer and any adjacent borehole section. Reference troughout this patent sPecification to a "c.-css-sect'onai" area is reference 'to the cross-secticnal area cl' the transm, ission (communication channel.) For another, a directicnaJ couPier arrangement i's described which 'Is at least partially responsible for inhibiting transmission opposite to the direction in the borehole of the desired communication. Specifically, a reflection section is defined in the borehole, which section is sPaced generally an odd number of quar-ter wavelengths from the transducer and positioned in a direction opposite that desired for the communication. to reflect back in the proper communication direction, any acoustic waves received by the same which are being propagated 'In the wrong direction. Most desirably, a muffi.ple number of reflection sections meetinc this criteria are provided as wifi be described in detail.

A speciai bidirectional coupler based on back-loading of the transducer piston also can be provided for this purpose. Most desirably, the borehoie acoustic communication transducer of the inventon has a chamber definine a compliant back-lead for the piston, through which a window extends that is spaced from the location at which the remainder of the transducer interacts with borehole liquid by generally an odd number of quarler wavelengths of the nominal frequency of the central wavelength of potential communication waves at the locations of said window and the point of interaction.

Other features and advantaces of the invention will be disclosed or,,iii become apparent from the following more detailed description. While such description includes many variations which occurred to APplicant, it be recocnized that the coverage afforded Applicant I's not!Imited to such variations. In other words, the presentation is suPpcsed to be exemplary, rather than exhaustive.

THE COMMUNICATION SYSTEM- The present Invention relates to a practical borehole acoustic communication system. It is capable of comm unicatinc 'n both flowing and non-flowing viscous liquids confined in a borehole, althouah many oi its features are useful in borehole communication with production tubing or a drill stem being the acoustic medium. Its desicn, however, takes into consideration the waveguide nature c;, a bcrehole.!'L has been found that to be practical a borehole acoustic communication system has to operate at frequencies below one kilohertz with an adequate bandwidth. The bandwidth depends on various factors,;.nc!udine the efficiency of the transmission medium. It has been found that a bandwidth of at least several Hertz are required for efficient communication n, varlous!liquids. The system must transfer information in a robust and relab!e manner, even during periods of excessive acoustic noise and in a dynamic environment.

As an ImPortant feature of the invention, the acoustic communication system characterizes the transmission channel when (1) system operation is initiated and (2) when synchronization between the downhole acoustic transceiver (DAT) and the surface acoustic transceiver (SAT) is lost. To facilitate the channel characterization, a wide-band "chirp" signai, (a signal having is energy distr;buted throughout the candidate spectrum) Is transmitted from the DAT to the SAT. 7he received signal Is processed to determine the per-Lion of the spectrum that provides an exceptional signal to noise ratio and a bandwidth capable of supporting data transmission.

As another Imponant eature of the invention, d provides r 1 two-way communication beween the locations. Each of the communication transducers is a transceiver for bo,,h,,eceiving acoustic signals from, and for imparting acoustic signals to, til-l-- (pre.erabiy) non-moving borehoile liquid. The conimunication Is reciprocai In,1.a., lt i's provided by assuring that the electrical load impedance for receiving an acoustic signal from the borehole liquid equals the source impedance cl' such transceiver for transrnitting. Most desirabiy, the transceivers are time synchronized to provide a robust communication system. Initial svnc,,roni'zati'on is accomplished through transmission of a synchronization signal in the form of a repetitive chirp sequence by one of 'the units, such as the downhole acoustic transceiver (DAT) in the preferred embodiment. T-ne surface acoustic transceiver (SAT) processes the received sequence lie establish approximate clock synchronization. When communca'ticnl's between a downhole locaflon and the surface, as in 'the preferred embodiment, it is preferred that most, not all, of tf' ie data processing take place at the surface where space is plentiful.

7nis first synchronization Is only an approximation. As another dominant feature, a second synchronization signal is transmitted from the SAT to the DAT to refine such synchronization. The second synchronization signal is comprised of two tones, each of a different frequency. Signal analysis of ti",lese tones by the DAT enables the timing of the DAT to be adjusted 'Into synchrony with the SAT.

Althouch the communication system 1 of the Invention S particularly designed for use of a borehole liquid as the transmission medium, many of its features are usable to improve acoustic transmission when the transmission system utilizes a drill stem, production tubing or other means ex-tending in a bcrehole as a Lransmission medium. For example, it provides clock correction during the 'Lime data Is being transmitted. Other features anc advantages of the invention. either will.

become apparent c,r be described in the following more detailed description of a preferred embod;ment and alternatives.

THE MEASUREMENT-WHILE-DRILLING APPLICATION: While the preferred embodiment of the present invention discussed herein is the utilization of the communication system in a produc:'ng oil and gas well, It is also possible to utilize the transducer and the communication system of the present invention during drilling operations 'to transmit data, preferably through the drilling fluid, between (1) selected points in the drillstring, or (2) between a selected point in the drillstring and the earth's surface. The present invention can be utilized in parallel with a conventional measurement-whiledrilling data ',,-ansm';ssion system, or as a substitute for a conventional measurement -wh ile-d rilling data transmission system.---Ine present invention is superior to conventionai i-reasurement-while-drilling data transmission systems Insciar as communication can occur while there is no circulation of fluid in the wellbore. The present invention can be utilized for the bidirectional 'transmission of data and remote control signals within the weilbore.

GAS INFLUX DETECTION. The transducer and communication system of the present invention can also be utilized in a wellbore to detect the entry 1 1 - of natural gas into the wellbore, typically during drilling and completion operations. As those skliled in the art will understand, the introduction of high pressure gas into a fluid column in the wellbore can result In loss of control over the well, and in the worst case, can resuft in a blowout of the well. Present technologies are inadequate [or determining both (1) that a undesirable eas influx has occurred-, and (2) the location of the gas "bubble" within the fluid column (bear in mind the cas influx wiii travel generally upward in the fluid cclumn). The present invention can be utilized to determine whether or 1-ot a gas bubble is present in the fluld column, and to provide a generai 'indication of the location of the gas bubble within the fluid column. With this information, the well cPerator can take precautionary measurements to prevent loss of control of the well, such as by increasing er decreasing the "velcht" (density) of the fluid column.

Additiona objectives, features and advantages will be apparent in the written description which follows.

BRIEF DESCRIPTION OF THE DRAWINGS

The novel features believed characteristic of the invention are set forth in the aPPended c!ajms. The invention ftself, however, as well as a preferred mode of use. furlher objectives and advantages thereof, will best be understood by reference to the following detailed description of an lilusirative embodiment when read in conjunction with the accompanying drawings, wherein.

Figure 11 is an overall schematic sectional view illustrating a potential location within a borehole of an implementation of the invention; Ficure 2 is an enlarged schematic view of a portion of the arrangement shown in Flioure 1 - Ficure 3 is an overall sectional view of an implementation of the transducer of the invention,' Ficure an enlarged sectional view of a portion of the construction shown In Floure 3; Figure 5. is a transverse sectional view, taken on a plane indicated by the lines 5-5 in Figure 4,' Figure 6 Is a partial, somewhat schematic sectional view showing the magnetic c:,,cui't provided by the implementation illustrated in Figures 3-5,- Figure 7A is a schematic view corresponding to the implementation of the invention shown 'In Figures 3-6, and Figure 7B is a variation on such implementation; (Figures 8 through 11 Illuwate various alternate constructions.

Figure 12 I'llust,rates In schematic lorm a preferred combination of such eements; 13 is an overall sectional view of another implemenlation of the instant invention; Ficure 14 is an enlarced sectional view of a portion of the construction shown in Ficure 13.

Figures 15AA5C illustrate in schematic cross-section various constructions of a directional coupler Portion of the invention.

Floure 16 is an overall somewhat diacrammatic sectional view illustratine an implementation of the invention, a potential location within a borehole for the same; Floure 17 is a block diagram of a preferred embodiment of the Invention', Figure 18 is a flow chart depicting the synchronization process of the downhole acoustic transceiver por-,.lon of the preferred embodiment of Ficure ';7.

Floure 19 Is a flow chart depicting the synchronization process of the surface acoustic transceiver port 1 on of the preferred embod 1 ment of Figure 2,- F'C-ure 20A, 20B, and 20C depict the synchronization signal st ru c t u r e.' Ficure 2 1, Is a detalied block diagram of the downhole acoustic transceiver., Ficure 22 is a detailed block diagram of the surface accustic transceiver', Fgure 23 depicts the second synchronization signals and the resultant correlation signals., Figure 24 depicts the utilization of the transducer and communication system in the present invePtion in a drillstring during drilling operations to transmit data between seiected locations in the drillstring; Figures 25 and 26 are utilized to illustrate the application of the transducer and communication system of the present invention during drilling operations for the purpose of identifying and detecna the influx of gas Into a wellbcre fluid column; and Figures 27 and 28 are block diagram representations of an alternative data communication system for the present invention.

DETAILED DESCRIPTION OF THE INVENTION

THE TRANSDUCER. The transducer of the Present Invention will be described with references to Ficures 1 through 15.

With reference to Figure 1, a bcrehoie, generally referred to by the reference numeral 11, 'is illustrated e)cending through the earth 12. Borehole 11 is shown as a petroleum product comPlelion hole for illustrative PurPoses. It indudes a casing schematically illustrated at 13 and production t,jbinc 14 within which the desired oil er ether petroleum product flows. The annu!ar space between the casino and production tubing is filled with a completion liquid represented by dots 16. ---ine viscosity of this completion!Ici,uld could be any viscosity within a wide range of possible visccsies. Its density also could be of any value within a wide range, and a may include corrosive liquid components like a high density salt such as a sodium, Potassium and/or bromide compound.

In accordance with c-onventional practice, a packer represented at 17 is provided to seal the bcrehole and the completion fluid 1 --l-,e croduction tubine 14 extends ---om 'he desired petroleum crcduc+. through the same as illustrated and rn a y include a safety valve, data gathering instrumentation, or other tools on the petroleum side of tije packer 17.

A carrier 19 for the transducer of the Invention is provided on the lower end of the tubing 14. As illustrated, a transition section 21 and one or more refilecting sections 22 (which will be discussed in more detail below) separate the carrier from the remainder of the production tubing. Such carrier indudes a slot 23 within which the communication transducer fthe s held in a conventional manner, such as by strapping or the like. A data gathering instOrument, a battery pack, and other components, also could be housed within siot 23.

it is the completion liquid,6 which acts as the transmission medium for acoustic,.javes provided by the transducer, but any other fluid can be utilized for transimission, including but not limited to production fluids, drilling fluids, or fresh er salt water. Communication between the transducer and the annular space which conflnes such liquid is represented in Figures 1 and 2 by port 24. Data can be transmirted through the port 24 to the completion liquid and, hence, by the same in accordance with the invention. For example, a predetermined frequency band may be used for signaiing by conventional coding and modulation techniques, binary data may be encoded into blocks, some error checking added, and the blocks transm tted serially by Frequency Sh;f-t Keying (FSK) or Phase Sht Keying (PSK) modulation. The receiver then wiii demodulate and check each block for errors.

The annular s.cace at the carrier 19 Is significantly smaller in crosssectional area than that of the greater part of the well containing, for the most par-t, only production tubing 14. This results in a corresponding mismatch of acoustic c.aracterisl;'c admittances. The purpose of transition section 21 is to minimize the reflections caused by the mismatch between the section having the transducer and the adjacent section. It is nominally one-quar-ter wavelength long at the desired center frequency and the sound speed in the fluid, and d is selected to have a diameter so that the annular area beCNeen it and the casing 13:S a geometric average of the product of the adjacent annular areas, ",',,at is, the annular areas defined by the production tubing 14 and 'the carrier 19). Further transilion sections can be provided as necessary in the borehole to alleviate mismatches of acoustic admit---tances along the communication path.

Reflections from the packer (or the well bottom in other designs) are minimized by the presence of a multiple number of reflection sec,.ons or steps below the carrier, the '',rst of which is indicated by reference numeral 22. lt provides a transition to the maximum possible annular area one-quarter wavelength below the transducer communicaton port. It Is foilowed by a quarter wavelength long tubular section 25 pid 1 1 1 imum cross-sectional area rovi ing an annular area for liquid with the m ini it otherwise would face. Each of the reflection sections or steps can be muiti,oie number of cluaner wavelengths Iona. The sections 19 and 21 should be an odd number of quarter wavelengths, whereas the section 25 should be odd or even (Including zero), depending on whether or not the last step before the packer 17 has a large or small cross-section. It should be an even number (or zero) d 'he last step before the packer is from a larcie cross- section to a small cross-section.

While the first reflection step or section as described herein is the most effective, each additional one that can be added the degree and bandwidth of isolation. (Both the transition section 21, the reflection section 22, and the tuLlar section can be considered as parts of the combination making up the preferred transducer of the invention.) A communication transducer for receiving the data is also provided at the location at which it is desired to have such data. In most arrancements this will be at the surface ol the well, and the electronics for operation of the receiver and analysis of the communicated data also are at the surface or 'in some cases at another location. The receiving transducer 24 most desirably is a duplicale in principle of the transducer being described. (It is represented in Figure 1 by box 25 at the surface of the well). 7he c--mmi-,ncation analysis electronics is rePresented by box 26.

I 'I be reccenized by 'hose skilled in the art that the acoustic t wl, - 1 1 1 transducer arrancernent of the invention is not limited necessarijy to communication from, dovinhoie to the surface. Transducers can be located for communication between Mo different downhole locations. It is also important to note tha, the principle on which the transducer of the invention is based lends itself to two-way design: a single transducer can be designed to both convert an electrical communication signal to acoustic communication waves, and vice versa.

An imolementation of the transducer of the invention Is generally referred to by the reference numeral 26 in Figures 3 through 6. This specific design terminates at one end in a coupling or end plug 27 which is threaded in,,,.), a bladder housinc 28. A bladder 29 for pressure expansion is provided in such housing. The housing 28 includes por-ts 31 for free flow Into the same of the borehole completion iiquid for interaction with the bladder. Such bladder communicates via a tube with a bore 32 extending through a coupler 33. The bore 32 terminates in another tube 34 which extends into a resonator 36. The lencth of the resonator is nominally A14 in the liquid within resonator 36. The resonator is filled with a liquid which meets the criteria of having low density, viscosity, sound speed, water content, vapor pressure and thermal expansion coefficient. Since some of these requirements are mutually contradictory, a compromise must be - 1 C made, based on the condition of the application and design constrai 1 1 1 L ints.

The best choices have thus far ben found among the 200 and 500 series Dow Corning silicone oils, refrigeration oils such as Capella B and lightweight hydrocarbons such as kerosene. 7he purpose of the bladder construction Is to enable expansion of such liquid as necessary In view of the pressure and Lemperature of the borehoie liquid at the devnhoie location of the transcucer.

-he ra: LIC j nsducerof the invention Generates (or detects) acous' waveenergy by means of the interaction of a piston in the transducer housing with the borehole liquid. In this implementation, this is done by movement of a piston 37 in a chamber 38 filled with the same liquid which fills resonator 36. Thus, the interaction of piston 37 with the borehole liquid is indirect: the piston is not in direct contact with such borehole liquid. Acoustic waves are Cenerated by expansion and contraction of a bellows type piston 37 in housing chamber 38. One end of the bellows of the piston arrangement is permanently fastened around a small opening 39 of a horn structure 41 so that reciprocation of the other end of the bellows will result in the desired exPansion and contraction cf the same. Such expansion and contraction causes corresponding llexures of isolating diaphragms 42 in windows 43 to impart acoustic energy waves to the borehole liquid un the other side of such diaphragms. Resonator 36 provides a compliant backload for this piston movement. It should be noted that the same liquid which fills the chamber of the resonator 36 and chamber 38 fills the various cavities of the piston driver to be discussed hereinafter, and the change in volumetric shape of ci-lamber 38 caused by reciprocation of the piston takes place before pressure equalization can occur.

One wav of lcokinc at the resonator is that its chamber 36 acts, in effect, as a tuning pipe Icr returning in phase to piston 37 that acoustical energy which is not transmitted by the piston to the liquid in chamber j8 when such ciston first moves. TO this end, piston 37, made up of a steel bellows 46 4), is open at th-e surrounding horn openinc 39.

-7he other end of the bellows is ctsed and has a driving shaft 47 secured hereto. 7ne horn S:r(-Icture 41, c---,r-,.unica,Les the resonator 36 wth the piston, and such resenatol aids In assuring that any acoustic energy generated by the pis,cn that does not directy result in movement of isoia'!'nci dlaphragms 42 wiiI reinicr.-e 'he oscillatory motion of the piston.

in essence, its intercepts 'tha7 aCOUS-' ic wave energy developed by the piston which does not drectly result in radiation of acoustic waves and uses the same to enhance such radiation. 11. aso acts to provide a compilant back!oad for the piston 37 as stated Previously. it should be noted that the inner wali of the resonator could be tapered or otherwise contoured to modify Lhe frequency resPcnse.

The driver forthe plszcn wiii now be descl,Ibed. It inc!udes the drivinc shaf-L 47 secured to the ctsed end of 'the bellows. Such shaft also is connected to an end Ca.0 48 for a tubular bobbin 49 which carries two annular coils or windines 51 and 52 in corresponding, separate radial gaps 5-53 and 54 (Figure 6) of a closed [coo magnetic circuit to be described, but a greater number of bobbins could be utilized. Such bobbin terminates at its other end in a second end cap 5.5 which is supported in position by a flat spring 56. SPring 56 centers the end of the bobbin to which it I's secured and constrains the same to ImIted movement in the direction of the loncitudinal axis of the transducer, re,--resen,ed in Figure 4 by line 57. A s' ilar flat sprng, s orov ded 'or the end cas 48.

imi 1 h 1 1 r,. kee-sinci with 'the inventien, a magnetic Clrcui- having a plurality of gaps is defined within the housing. TO this end, a cylindrical permanent magnet 60 is provided as par of the driver coaxial with the axis '57. Such permanent magnet gene,-ates the magnetic flux needed for the magnetic circuit and 'L;-:rminates at each of its ends in a poie piece 61 anc 62, resPectively, to concentrate the magnetic flux for Tlow through the pair of longitudinally spaced apart gaps -53 and 11-5.4 in the magnetic circurt.

magnetic c;rcull Is com, pletec by an annular magneticaily passive member of macnetically permeable matenal 64. "s iiustrated, such member includes a pair of inwardiy directed annular ilances 66 and 67 which terminate adjacent the windings 51 and 52 and define one side of the gaps 53 and 54.

The magnetic circuit forr-ned by this implementation represented in Figure 6 by closed loop magnetic flux lines 68. As illustrated, such lines extend from the magnet 60, through pole piece 6 1, across gap 53 and coli 51, through the return path provided by member 64, through gap 54 and coil 5-52, and through pce piece 62 to magnet 60. With this arrancement, It will be seen that magnetic flux passes radiaily outward t,rouc, 1-ac -3 and radia!y inward gap 54- Coils 5.1 and 52 are connected in series opposition, so tna., c,,rent in the same ph ovides additive lorce on the common bobbin. Thus, if the transducer is being used to transmit a communication, an electricai signal defining the same is passed throucin' the coils 51 and 52 will cause corresponding movement of the bobbin 49 and, hence, the piston 37. Such piston will interact through the windows 43 with the borehole liquid and impart the communicating acoustic energy tillereto. Thus, the electrical Power represented by the electrical signal is converted by the transducer to m echanical power, in the form of, acoustic waves.

When the transducer receives a communication, the acoustic energy deflining the same will flex the diaphragms 42 and correspondingly move the piston 37. Movement of the bobbin and windings within 'the gaps 51 and 52 will generate a corres,cendirc electrical signal in the coils 51 and 52 'in view of the lines of m, acn.-tc -lux.rilch are cut by the same. In other words, the acoustic: power is to electrical Power.

In the be!ng desc.,lbed, it vilil be recognized hat the permianent -,..agnet' 60 and zs associated pole pieces 61 anci 62 are ceneraliv c,iindrical in s,ac)-- with 'he axis 57 actinci as an axis oi a 'cgur- of revolution. The bobbin is a cylinder with the same a-xis, with the coils 51 and 52 beng annular in shape. Return.calth member 64 also is annular and surrounds the rnagnet, ei.-. The is he!d centrally by support rods 711 projecting 1 nwardly fli-cm, the return Path member, through slots In bobbin 49. The flat sPrIng-s 56 and 58 -,--,rresp,-,ndincly centralize the bobbin while allowinc limited loncitudinai r7,ctlcn of thie same as aforesaid. Suitable electrical leads 72 I'or the windincs anc other electrical parts pass into the S 73.

housing through cc-,ed 1'eedt.,rouch i r-JG 7A illustrates te 'm piementaticn described above in schemi atic iorm. The resonator is represented at 36, the horn structure a: 41, and the pliston at 37. TIne driver shaft of the piston is represented at 47, whereas the driver mechanism itsef is represented by box 74. Figure 7B shows an alternate arrancemient in wnich the driver is located within the resonator 76 and the Piston 37 communicates directly with the borehole iquid which is allowed to 'llow In,hrLch windows 43. -Fre windows are open.' they do not include a diachracm cr other structure which prevents 'the borehcle from en',-nine t1e c-.a-L-.er 38. It will be seen that in this arrangement the pston 37 and the horn structure 41 provide fluid-tight isolation between such chamber and the resonator 36. It will be recognized, though, that it also could be designed flor the resonator 36 to be flooded by the I--crehoie liquid. It I's desirable, if it is desicned to be so flooded, that such resonator indude a small bore filter or the like to exclude suspended par,Lides. In any event, the driver itseif sinculd have 'Its own inert fluid system because cl; close tolerances, and st.rong magnetic ','ieids. 7ne necessary use of cer',aln materials In the same makes It prone to impairment by corrosion and contamination by panicles. Par'ticularly magnetic ones.

Figures 8 through 12 are schematic 1!1ustrations representing various conceptual approaches and m, odifications for the invention, considered by applicant. Ficure 8 illustrates the modular design of the!nvention. In this connection, it should be noted that the Invention is to be housed in a pilpe of restricted diameter, but length is not critical. The invention enables one to make the best Possible use of cross-sectionai area multiple modules can be stacked to 'Improve efficiency and Power capability.

The bobbin, represented at 8,11 'In Figure 8, carries three separate annular windings represented at 82-84. A pair of magnetic circuits C. I'ded, with permanent magnets represented at 86 and 87 with facing are Pr v 1 L magnetic polarities and poles 88-90. Ret.urn paths for both circuits are provided by an annular passive member 91.

It will be seen that the two macnetic circuits of the Figure 8 configuration have the central pole 89 and its associated cap in common.

7ne result Is a three-coll driver wth a efficiency (available - 2 0 acoustic power ou.,-,ut/elec,,ic power input) greater than twice that of a sincie driver, because of the absence of fringing flux at the joint ends. Obviously, the process of "s'Lacking" two coil drivers as indicated by this arrangement wiih a!,ernat:ng rnagnet polarities can be continued as long as,-,'es'red wi:h '-,e c-,.-7r-,on bobbn be 1!ng approprIately sucpcred. In this scr,,em,atic arran-ement, the bobbin is connected to a pstcn 85 :nc!udes a cenLai c--.,-ned Par-t and bellows of tJ-,e like sealing the sam, e lo an outer casino Irenr:zsented at 92. -inis flexure seal support is preferred c sild'lnc seals anc --ear,7gs because -,e Iater exhib res'r'[&,lon ma, n,,rcduced -ar-icLiar;lv at the sm, all disclac-cments encountered when the transduce,r Is used for receiving. Alternatively, a rigid piston can be sealed to the case with a beilows and a separate spring or spider used re- tr c-nterinc. A sP, -resented at 94 can be Lsed at the coposce end of the bobbin for centerinc the same. if such sPider is metal, i-t can bensL, iated from, the case and can be used or electrical connections to the moving windings, emiratinc the flexible leads otherwise required.

In the aiternative schemallically ilillustrated in Ficure 9, the m, acnet 86 iS rrac:- a:",-ju,ar and t surrounds a Passive 'iiux reTurn Path member 91 in jzs --enter. Since passive materials are available wh saturatien flux densi:jes about twice the remanence of magnets, tine design illustrated has the a-vantace of allow ing a small diameter of the poles represented at 88 and 90 ',c reduce wil resistance and increase efficiency.

The Passive flux path member 9 1 couid be replaced by another permanent magnet. A m, acinet design, cl' course, could peri-nd a reduction in length cl;,,e driver.

-,z S- - Figure 10 sc,nemati:ca!iy iil6,sL,-altes another magnetic structure Icr the driver. 1, indudes a pair of oppositely radially polarized annular magnets 95 and 96. As illustrated, such, magnets define the outer ed 1 ges of 1 is the gaps- In this ariancement, an a,-,,ruar passive magnetic mer-nder 97 1 Provided, as,,eil as a central re-lurn path member 91. While this arrancer-nenz has ',il-le ac-vantace cl; eng,,, due to a reduc.ier, cl', ux eakace at tihe ca,cs anc lc,,,i ex,,er,-,al;ILx leakage, it has the disadvantace of mcre rnaciriel 1;abric-.=z;on an.d lower flux density in such caPs.

in,.er-,aces be Prov:ded between the magnets and pole pieces. Thus, thle miatinc J'unc,,lcns can be made oblique to the long axis of th'. transducer. This construction maximizes 'the magnetic volume and its accom panvinc available avoiding localized flux densl"Lies that could exceed a macne, remanence.;, should be noted that any of the Junctions, mac net, cole Piece-to-pole piece and of course magnet-to-Pole Plece can be made winical. Figure 11 lilusiraes one arrangement fOr j;e-7.,.,re. t should be noted that in this arrancernient tl^ie magnets may inc!udes pieces 98 a, the ends of the passive filux return member 9 1 as ';iius,.,atcd.

Figure 12 schematicaily illustrates a par-ticular combination of 'L,e options set 'or,,-, in Figures 8 thcrough 11 which could be considered a preferred embccimer-,L lor ceria!n applications. it inc!udes a pair cif pole pieces 101, and 1C2 mate con:--aliy with, radial magnets 103, 104 and i i 1 05. The Ne r-,,acne,"c crcuits c, are formed include passive return path members 11C16 anc 1107 terminaiing at the gaps in additional magnets 108 and 110.

An Implementation of the Invention incorporating some of the features mentioned above is Illustrated In Figures 13 and 14. Such implementation Includes two magnetic G!rcults, annular magnets defining the exterior of the magnetic circuit and a central poie piece. Moreover, the piston is in dire-t contact with the borehoie 1 qu d and the resonant chamber is filled with such liquid.

The Implementation shown in Ficures 13 and -14 is similar in m any aspects 'to the implementation 1Hustrated and described with respect to Figures 3 and 6. Common pans will be referred to by the same reference numerals used earlier but with the addition of prime component. This implem, entation Includes many of the features of he earlier one, which features should be considered as beine, incorporated within the same, unless indicated otherw'se.

The implementation of Figures 13 and 14 is generally referred to by the reference numeral 1 -1-0. The resonator chamber 3.6' is downhole of this piston 37' and its driver, in this arrangement, and is allowed to be 2led with bcrence liquid rather than being filled with a special liquid as described in connection with the earlier Implementation. The bladder and i-ts associated housing is eliminated and the end plug 27' is threaded directly into the resonator chamber 36. Such end plug includes a plurafty of elongated bores 122 which communicate the borehole with tube 34' exlending in to the resonator 36. As with the previously described implementation, the tube 34' is nominally a quar-ter of the communication wavelength long in the resonator fluid (the borehole liquid in this implementation). The diameter of the bores 122 is selected relative to the interior diame!c-r of tube 34' to assure Lhat not Particuiate matter from the borehole liquid which is of a su,.j:ic'ienz!y large size to block such 'tube will enter the same.

It will be re-loonized while wIth this airancierrient the damber 36' which Provides a compliant backload for rnovernent of the piston -17' Is in direct communication wIth the bcrehole liquid thrcugh t h e 1 ItL ,ube 34', acoustic wave energy in the sarne w fl not be tr a n s.-P,'- e d t c the x4 1 Ler c( of ',he charnber because oi a7..er-,La,'on by such tube.

Piston 37' Is a bellows as described in the earlier imPlennentation and acts to isolate the driver for the same to be described wth the borehoie 1'cud.

trom a cham ber 38' which allowed to be 0 1 1 1 Such chamber 38' is lilustrated as having two Parts, pai-:s 123 and 124, that communicate directiv with one another. As Illustrated, windows 43' extend to the annulus surrounding the ti-ansducer construwicn without the intermediary of Isolating diaphracm, s as in the previous implementation. Thus, in this Implementation the piston 337' is in direct contact with borehole liquid which fills the chamber -18'.

The Piston "7' is connected via a nut 127 and driving shaft 128 to the driver mechanism. To this end, the driving shaft 128 is connected to an end cap 48' of a tubular bobbin 49'. -Fne bobbin 49' carries three annular coils or winclings in a corresponding num ber of radial gaps of two closed loco m, agnetic circuits to be descited. Two of these windings are represented all, 128 and 129. The third winding is on the axial side of winding 129 opposite that of winding 128 in accordance with the arrangement shown in Floure 8. Moreover, winding 129 is twice the axial length of winding 128. The bobbin 49' is constrained in position similarly to bobbin 49' by springs 56' and.58'.

The driver in,'71s!m, conceptuallyis a hybrid of the ac)oroac-,es lilustraed in Ficures 8 and 9. 7nat is, It includes mc adiacent -,,acnetic circuits sharinc a comimon pathway. Moreover, the per, manent rnagnets are annular a solid core providing a passive member.

In more detail, 'three magnets liluszra'ed in Ficure 14 a' '131, 132 and 13,3, de,ec.o flux which fiows across the caps within which the winclings previously described ride to a sc':id, cyindric-ni core passive member 132.

T-ne magnetic circuits are com iered by an annular casing 134 which p 1 surrounds the magnets. Such casino 134 is fluid tight and acts to Isolate ,he driver as described from the ccrehole liquid- In this connection, it Includes at its end Caced from pis-,on 37', an isolation bellows 136 which transmits pressure chances caused in the driver casino 132 to the resonalor 36'. The bellows 136 is free floating In the sense that ft is not physically connected to the tubular bobbin 49' and sim.ciy flexes to accommodate the pressure chances of the special l-juld in the driver casing. It sits wdhin a.

central cavity or boirehcle 37 a pluc -38 Lal e)',ends between the driver casing and the vall cfl the rz-scnant chamber 3C6V. An elongated hole or aperture 139 connects 1he ln,erior of be!loNs 1316 with the resonator chamber.

A passive directional coupling arrangement is conceptually iillustrated by Figures 15A-1 5C. 7ne piston of the transducer is represented L 1 '1 at 220. Its design is based on the fact that, the acoustic charac:teristic admittance in a cylindrical waveculde Is p(o.portional to its cross-sectional 7ne windows for transm ss on, c); the comm, un cating acoustic energy area. 1: i 1 1 1 1 to the borehoie fluic are represented at 221. A second port or annular series of ports 222 are located either three one-quarter wavelength section (Ficure 15A) or one-cluarter wavelength sections (Figures 15B and C) from the windows 221. 7ne couPler is divided Into three quarter wavelength sections 223-226. The c.-css-se--tional area of these sections are selected to minimize any misr-natch micht defeat directional coupling. Center section 224 has a cross-sectional area A. which Is nominally equal to the square of the cross -s ectional area of sections 223 and 226 (A.) civided by the annular cross-section ot tje borehole all the localion of the porls 221 and 222. 7ne reduced cross -sectional area of section 224 is obtained by including an annular restricticn 227 'in the same.

7ne directionai coupler Is in direct, contact with the backside of the piston 220, with the result tha: acoustic wave energy will be introduced into the coupler which is 1800 out-of-phase with that of the desired communication. The relaiions,l.o of the cross-sectional areas described previcusiy will assure that the accustic energy which emanates from the pert 222 wiii cancel any transrnissicn from port 221 which otherwise would travel toward Por-L 222.

The version of the directional coupler represented in Figure 15A is fuil length, requiring a three-quarter wavelength long tubing, i.e., the chamber is divided into three, quar:er-wavelength-long sections. The versions represented in Figures 15B and l 5C are folded versions, 'thereby reducing the iength required. That 'Is, the version in Figure 158 is folded once with 'the sectional areas of the sections meeting the crheria discussed previcLisy. Two of the chamber sections are coaxial with one another. The version represented n Ficure 15G folded Mice. That all three sections are coaxIal. The two versions in Ficures 15B and 15C are one-fourth wavelength from the port 222 and thus are on the "uphole" side of port 221 as illustrated. It will be recognized, though, that the bandwidth of effeotirve directional coupling is reduced wIth folding.

It will be recognized that in any of the configurations of Figures 15A-1 5C. the port 222 could contain a diaphragm or bellows, an expansion chamber could be added, and a filling fluid other than well fluid could be used. Additional contcuring of area could also be done to modify coupling bandwidth and efficlency. Shaping of ports and arraying of multiple ports could also be done for the same purpose.

Directional coupling also could be obtained by using two or more transducers of the invention as described with ports axially. separated 'to synthesize a phased array. The directional coupling would be achieved by driving each transducer with a signal appropriately predistorted in phase and amplitude. Such active directional coupling can be achieved over a wider bandwidth than that achieved with a passive system. Of Course, the predistor-ticin functions would have to account for all coupled resonances 'in each par-ticular situation.

THE COMMUNICATION SYSTEM: The communication system of the present Invention will be described with reference to Figures 16 through 23.

With reference to Figure 16, a borehole, generally referred to by the reference numeral 1100, is illustrated extending through the earth 1102. Sorehole 1100 is shown as a petroleum product completion hole for Illustrative purposes. it includes a casing schematically illustrated at 11 G4 - -l- and production tubing 1106 within which the desired oil or other petroleum product flows. The annular space be,.een the casing and production tubing is filled,ith borehole completion liquid represented by dots 1108. I he properties of a com, pletion fluid vary significantly from well to well and lt typi over time in aniv s-,ce,!;, ically will include suspended Panicles or par---tially be a ce.!' is r-,cn-Nevvtonian and may include non-linear elastic proPenties. its visccsiry could be any viscosity within a wide range of ible vsc-s't' 1 1 possi i - i;es. I's dersitv also could be o any value within a wide range, and it may in--!Ljde corrosive solid er liquid components like a high density salt such as a sodium, calcium, potassium and/or a brornide compound.

A carrier 1112 for a downhole acoustic transceiver (DA-n and hs associated transducer is provided on the lower end of the tubing 11106. As illustrated, a transition section 1114 and one or more reflecting sections 1116, most desirably are included and separate earner 1112 from the remainder ofProduction tubing 1106. Carrier 1112 inc!udes numerous slots in accordance wilh conventional practice, within one of which, slot 11 8, the communication trarsducer (DAT) of the Invention is held by strapping er the like. One or more data gathering instruments or a bartery pack also could be housed within slots like slot 1118. In the preferred embodiment, one slot is utilized to house a battery pack, and another slot (slot 1118) is utilized to house the transducer and associated electronics. It will be appreciated that a Plurality of slots could be provided to serve the function of slot 1118. The annular space between the casing and the production tubing is sealed adjacent the bottom of the borehole by packer 1110. The producton tubing 1106 extends throuch the packer and a safety valve, data gathering instrumentation, and other wellbore teols, mnay be included.

It is the Completion liquid 1108 which acts as the transmission medium for acoustic waves provided by the transducer. Communication between the transducer and the annular space which confines such liquid is represented 'in Ficure 16 by port 1120. Data can be transmitted through the porl 1 J20 to the cc ni.cietion liquid via acoustic signals. Such communication does not rely on flow of the completion liquid.

A sunface acoustic transceiver (SAT) 1126 is provided at the surface, communicating withthe cornpletion liquid in any convenient fashion, but preferably utilizing a transducer In accordance with the present invention. The surface configuration of the production well is diagrammatically represented and inc!udes an end cap on casing 1104. The production tubing 1106 extends through a seal represented at 1122 to a production flow line 1123. A flow line for 'the completion fluid 1124 is also illustrated, which extends to a conventional circulation sysiem.

In 'its simplest form, the arrangement converis information laden data into an acoustic signal which is coupled to the berehole liquid at one location In the borehoie. The acoustic signal is received at a second location in the borehole where the data 'is recovered. Alternatively, communlcdtion occurs between both locations in a bidirectional fashion. And as a fuCier alternative, communication can occur between muftiple locations within the borehole such that a network of communication transceivers are arrayed the borehoie. Moreover, communication could be through the fluld In the production tubing through the product which is being Produced. Many of the aspects of the specitIc communication method desc,,bed we applicable as mentioned previously to communication through other transmission medium provided in a borehcle, such as 'in the walls of the tubing 1106.

Referrina to Ficure 17, 'the downhole acoustic transducer (DAT) 1200 at 'the dr.,,,vnhole iccation is coupled to a dcwnhole acoustic transceiver (DA T) data acquisition system 1202 for acoustically transmitting data collected from the DAT's associated sensors 1201. -Fne downhole acoustic transceiver (DAT) data acquisition system 1202 includes signal processing circultry, such as impedance matching circuits, amplifier circuits, filter circuits, anaiog-to-dicital conversion circuits, power supply circuits, and a microprocessor and associated circuitry. The DAT 1202 is capable of both modulating an electrical signal used to stimulate the transducer 1200 for transmission, and of demodulating signals received by the transducer 1200 from the surface acoustic transceiver (SAT) 12G4 data acquisition system. The surface acoustic transceiver (SAT) data acquisition system 1204 inc!udes signal processing circuitry, such as impedance matching circuits, ampillier circuits, filter circults, analog - to-d ig Ital conversion circu s power supply circuits, and a microprocessor and associated circu". In other words, the DAT 1202 both receives and transmits information. Similarly, the SAT 1204 both receives and transmits information. 7n e communication is directly between the DAT 1202 and the SAT 1204 through transducers 1200, 1205. Alternatively, intermediary transceivers could be positioned with.in the borehoie to accomplish data relay. Additional DATs could also be Pro,.icled to transmit Independently gathered data from their own sensors to the SAT or to another DAT.

More specifically, the bi-directional communication system of the 'nvent on establishes accurate data transfer by conducting a series of steps designed to characterize the borehoie communication channel 1206, choose the best center frequency based upon the channel characterization, synchronize ',he SAT 12C4 with the DAT 1202, and, finally, bidirectionally t, ansfer data. This complex process is undertaken because the channel 1206 throuc,,7 which the acoustic sicnai must propagate is dynamic, and this tIme variant. Furiherrnore, the channei is forced to be reciprocal: the transducers are electncally loaded as necessary to provide for reciprocity.

In an effort to mitigate the effects of the channel interference upon the information throughput, the inventive communication system characterizes the channel in the uphcle direction 1210. To do so, the DAT 1202 sends a repetitive chirp signal which the SAT 1204, in conjunction with its computer 1128, analyzes to determine the besi center frequency for the system to use for effective communication in the uphoie direction. Currently, the channel 1210 is characterized only in the uphole direction; 'thus, an implicit assumption of reciprocity is incorporated into the design. It w51 be recccnized that the downhole direction 1208 could be characterized rather than, or in addition to, characterization for uphole ccmmunicat!cn. Moreover. in the current desion, the bit rate of the data transmitted by the DAT 1202 may be higher than the commands sent by the SAT 1204 to the DAT 1202. Th-,s, it is advantageous to achieve the best signal to noise ratio for the uphole signals.

Xternatively, if reciprocity is not met, each transceiver could be designed to characterize the channel in the incoming communication direction- zhe SAT 1204 could analyze the channel for uphoie,-ommuniction 1210 and the DAT 1202 could analyze for downhole communical,on 1208, and then command the corresponding transmitting - - I'D system to use the best center for the direction characterized by it. However, this aiternative would recuire ex:ra processing capability 'in the DAT 1202Ext,,a pr,-c,-ssi,-c calcalui,;i,,., means creater power and size requirements which are, In rnes', instances, undesirable.

In adeien to c-icosing a proper channel for transmission, syste.m ti ing is!r-t,cor-,ant to any coherent communication syslem. To acl---7,,,clls,-: ',,e channei cj"iarackerl'z-::: Lion and timinc synchronization Processes e DAT becins transi-nting repe'titve c,irp sequences a, 1 e- a procrammed le delay selected to be longer than the expected lowerinc tir-ne.

Figures KA-C delpict tne signalling structure for the chirp sequences. In a preferred Implementation, a single chirp block is one hundred milliseconds in duration and c,,--,ntains three cycles of one hundred fir 1 HIty (150) Hertz signal, four cycies c. two hundred (200) Hert:z signal, five cycles of ttwo hundred and flity (25SC) Herli signal, six cycles of three hundred (300) Her 7 signal, and seven cycles of three hundred and fi-fty (350) Hertz cycles.. The chirip s';cna structure is depicted in Fgure 20A. 7nus, the entire bandwidth of the desired acoustic channel, one hundred and fifty to three hundred and fifty (150-3-50) Hertz, Is chirped by each block- As de,c!cted in Ficure 20B, 'the chirp block is repeated Mh a time delay between each block. As shown In Figure 20,'this sequence is repeated three times at two minute intervals. The first two sequences are transmitted sequentially without any delay bemeen 'them, then a delay is created before a,,;rd sec.uence is transmitted. During most of the remainder of the intervai, the DAT 1202,,al's 1'or a command (or default tone) from 'the SAT 1204. The sPecific sequenc_e of ch'lrp signals should not be construed as limiting the Invention: variations on the basic scheme, inc!ud,;ng but not lirrItec t) di,ere,7 ch r- es, chirp durati 1 1, 1 joins, chirp jons, etr,.,,-e fores;=eadie. it s also contemplated t.at PN pulse se.carat sequences, an [m,.-ulse. cr any,,arable sicnal occupies the desired spectrum could be usec.

SAT 12G4 o:l ','-,e oreferred ernbodiment of the invention uses two micro Proces sc,,s!SIC-), 1626 to el;iectiveiy control the SAT iunctions, as is illustrated n Ficure 22. The host computer 1 128 controls all of the activities of the SAT 12n,4 and is connected thereto via one of two serial channels of a Mcdei 68OCIC r-,,ict,corocessor 1626 'in the SAT 14T- 4.

In alternative embodir-rents, SAT 120,4 m, ay be mounted on an inpL!/out. cut card which is adalsted in S'[ ze to be inserted within an expansion slot cf a host comPu-ter. -ine 68000 microprocessor accomplishes 'the bu, '^, cf',,'7e sicnal.orocessing 1;unctions that are discussed be!ow. 7ne second serial C2nnel of 'the 68C00 microprocessor is connected to a 68HC1 1 crccess,r 11 6 16 that controls 'the signal cligitization, the retrieval of received daa, and 1he sendinci cl; tones and commands to the DAT. The chirp sequenc-e is received from the DAT bi the transducer 1,205 and converted into an elecIrical signal 11rorn, an acoustic signal. TIne electrical signal is couPied to 'he eceiver 'through transformer 1600 which provides impedance r-,,atcninc. 1,11m.cliier -11602 increases the signal level, and the bandcass fliter 1E04 limj's the noise bandwidth to three hundred and flifty (350) Hertz centered at hundred and fifty (250) Hertz and also functions as an anti-alias fliter. 01 course, differel,It or additional bandwidths bet.ween as iarce as cne 'c as small as one Hertz could be utilized in alternative em bodm.en,,s of the present but for purposes of this written description, the range of lrequencies between one hundred Hertz and three hundred Her= will be discussed and utilized as an example, and not as a limitation cl' the present invention.

Refenin.- 'to Ficure 21, the DAT 1202 has a single 68HC11 microprocessor 15 12 that controls all transceiver functions, the data logging activities, logged data retrieval and transmission, and power control. For simplicity, all communications are Interrupt-driven. In addition, data from the sensors are buffered, as represented by block 1510, as arrives. Moreover, the commands are processed in the background by algorithms 1700 which are specifically designed for that purpose.

The DAT 1202 and SAT 1204 include, though not explic,ty shown in the block diagrams of Figures 21 and 22, all of the requishe microProcesscr Support circuitry. These circuits, including RAM, ROM, clocks, and bu'liers, are well known in the art of microprocessor circud design.

Genera'ion of the chirp sequence is accomplished by a digital signal generator controiled by the DAT microprocessor 1512. lLpic<-Afy,,,Lhe chirp block is generated by a digital counter having ds output controlled by a microprocessor to generate the complete chirp sequence. Circus of this nature are widely used for variable frequency clock signal generation. The chirp generation circuitry is depicted as block 1500 in Figure 21, a block diagram of the DAT 1202- Note that the digital output is used to generate a three level sicnal at '1 502 for driving the transducer 1200. It 'Is chosen for this application to maintain most of the signal energy In the acoustic spectrum cf, interest,' one and flitv Hertz tO three hundred and firty Hertz. The Pr;m, ary purlpose ot- the third s',a'Le 'Is to terminate operation of the transml-LL,lng Por-,Ion ol a transceiver dunng its receiving r.,ncde. it is, in essence, a c:rcj!t.

-icure and F cure 19 are;1',cw charts of 'he DAT and SAT operations, 1,escectivey. The c,io sequences are generated during step 1300. Prior 7C i,e f':rsz chirp pJse de!nci tr;-:nsmirled after the selected tir-re delay, the transceiver a.ai-,s the arnval of the sequences 'in accordance with ste-P '-100 in Ficure 19. -1he DAT is programmed to transmit a buist ef every NO Mlinutes until h receives Cwo 'tones.. ic and Illc + 1. Initial svnc,ronization s'ar-,,'s after a "characterize channel" command is the hos: UPon,,eceiving 'the 'charactenze channel" cor-,,r-,,and.. 'he SAT sarLs dicitizing transducer data. 7ne raw transducer data is conditicnec a chain of amp1flers, anti-allasing iiiters, and ievel before being d:c fized. One second data block (1024 sa,,7.p!es) 'Is stored In a bui,el, and pipelined for subsequent processing.

T',-,e of ',,he cjlr-- correlator are 'L.reefold. First, L synchronizes the SAT TX/RX clock to that ol the DAT. Second, r: calculates a clock error be,,jeen the SAT and DAT timebases, and corrects 'the SAT clock to matc. 'hat c'i the DAT. Third, It calculates a one Hertz resolution channel s,cectrL;r,-,.

7ne corr=-lator per-forms a FFT CFast Fourier Transform") on a.25 second bicc.'-, and retains FFT signal bins between one hundred and forty Her-,z c three hundred and sixty Hertz. -Fne complex valued signal is added coheren,..!, 'c a runninci sum bu,er containing the FF-i sum over the last six seconds (24 FF-js). In addition, 'he FFT bins are incoherently added as 'llcllcws. magnitude squar-ed, to a running sum over the last 6 seconds. An es,".-,ar-- cl' ',;-le sicnal to. nclse - at:'o (SNR) in each frequency bin is made bV a ra710 cjf zine coherent bin pcver to an estimated noise bin power. ihe ncise sower In each.-equency bin is compi-ned as the dl'i'ierence cl' the bin power m nus z-,. coherent bin -ewer. After 1 ' 1 1.- Iki I is co mouteC an 'SNR sum is c.--,mpijled by he SNIP n eac- d'n summInc the bin SNRs. 7ne SNR sum 'Is added,c he past twelve and eichteen second SNR sums to lorm, a correlatcr cLa,cut every.25 seconds and is n an eichteen second c:rcular bLrffer. In addition, a phase ancle In e-5z,--i frec.Lency bin is calculated from the six second butfer surn and Placed in, c an eichteen second c:',-cLlar.chase angle buffier for later use in c!cck error caiculations.

Af,er the chiro correlator has run the required number of seconds cl' daza '-,lrcuch and stcred the results in the correiator butfer, the correlator Peak is found '-nv comparing each correlator point 'Ice a noise floor P! u s a Pr e s e s t,; c i d. After dec-ctnc a ail subsequent SAT activities are synchr-nn';zed to 'the time at which 'kille Peak was found.

At'er the chirp presence is detec:ted, an estimate of sampling clock difference be,,Yeen the SAT and DAT is compLrted using the eighteen second c;rcular -hase ancle buffer. Phase ancle difference (nq5) over a six second time Intenial is computed for each frequency bin. A first clock error estimation is com...cu-,ed by averaging the weichted phase angle ciffierence over all the frequency bins. Second and third clock error estimations are Similarly calculated, respectively over tweive and one hundred and eighty-five - ('c- second tim e intervals. A welchted average of three clock error estimates gives the final c!ock error value. At this point in 'Lime, "the SAT clock is adjusted and further clock refinement is made at the next two minute chirp interval in similar fashion.

After the seccnd cioc,,,, re1nement, the SAT waits for the next set of chirps at the 1-wo minute interval and averages twenty-flour.25 second chirps over the next six secc,-,ds. The averaced data is zero padded and then FF-i is computed 'to Provice one Hertz resolution channel spectrum. 7-ne surface system looks for a suitable transmission frequency in the one hundred and fifty Hertz 'to three hundred and fifty Hertz. Generally, a frequency band having a oced signal to noise ratio and bandyAdths of approximately two Her-Lz to icr-.y Hertz Is acceptable. A width of the a[able channel defines the acc-oable baud rate.

val 1 -1, The second phase of the initial communication proc-ess involves esiabitshinc an operational comm, unication link between the SAT 1204 and the DAT 1202. Toward +,his end, two tones, each having a duration of seconds, are secuentlally sent to the DAT 1202. One tone is at the chosen center firequency and the other is offset from the center frequency by exactly one hertz. This step in the operation of the SAT 1204 is represented by block 1406 in Figure 19.

The DAT is aiways looking for these two tones: fc and fc+ 1, after it has stopped chirping. Before lookinci for these tones, acquires a one second block cl' data at a time when rt is known that there is no signal. The noise collection generaliv starts six seconds after the chirp ends to provide time for echoes to die down, and continues for the next thirty - 1 seconds. During the thir-,y second noise collection Interval, a power spec,,,um of one second data block is added 'to a three second long running averace power spectrum as o,en as the processor can compute the 1024 point (one second) power spectrun,.

The DAT starls lcok;nc for the two 'tones approxi mately thirtysix se,.nds a'Le.,,he end cl the c-,ir,o and continues looking for them for a perled of four seconcs (tone dur-:::,,jcn plus twice 'the maximum propagation timie. -ine DAT acain caicLiates power sPectrum cii one sewnd blocks as fast as It can, and computes signal to noise ratios for each one Hertz w'de f,,ecuencv bins. All the frequency components whic are a preset I ki,res,cld above a noise floor are Posstle candidates. If a frequency is a candidate in mo successNe b!ccKs, then the tone is detected at Its frequency. 11 the tones are not reccenized, the DAT continues to chirp at the next two minute interval. When the tones are received and properly rec--cnIzed by the DAT, the DAT transmits the same two tones back to the SAT -' 'the selected carrier -1,-,.c,uency fc, which is recognized as an ac.l.<nowledeement sicnal. Then, t11-1e SAT transmits characters to the DAT, vihic, causes the CAT to Icck lor a coded "recognition sequence signal".

Conruci data follows the recoeniticn sicnal. Preferably, the recognition sequence signal inCudes a baud rate sigi ial which identifies to the DAT the expected baud rate, as determined by the SAT. The DAT will then respond to any command provided to It after the recognition sequence signal.

Typicaily, the SAT will comm, and t,e DAT 'to begin the 'transmission of data ,crn ' 1 i, he downhcle location for rece pt by the SAT at the uphoie location.

A by-product of the p(ocess of recognizing the tones is that rt ena.-ies ',he DAT to synchronize s 'InkernaJ clock to the surface -4 2- i 's clock. Using ',,'-,,e SAT clock as the reference clock, the tone transce ver pair can be said to becin at tinne ',=0. Also assume that the clock in the surface transceiver produces a tick every second as depicted in Figure 23.

i nis allenment I's Cesirab!e to enable each clock to tick off seconds synchronously and mainzain cor-,erec, for accurately demodulating 'the data. However, 'L,-,,e DAT 's not when It will receive the pair, so it conducts an FFT even se=nd c Its own internal clock which can be assumed net,c c_= aioned 'he sur-face clock- Wh e n t h e '1c u r seconds of tone Pair arrive, they more than likely cover only thre-e one second FFT interval fu(iv and oniv.vo of those will contain a single frequency. Floure 23 'Is helpful in this arrangement. Note that the FFT periods having a fuil one second of lone signal located wrkhin it will produce a maximum, FrT peak.

Once received, an FF-7 of each two second tone produces both amplitude and phase components of the signal. When the phase component of the sicnal is comIsared with the phase component of the second sicinal, the ore second ticks of the downhole clock can be aligned with the sur-ace clock- For examPle, a t-wo hundred Hertz tone lcllowed immediately by a rovo hundred and one Her= tone is sent from the transceiver at time t=0. Assume that the propagation delay is one and onehalf seconds and the difference between the one second ticking of 'the clocks 'Is.25 seconds. This interval s equivalent to three hundred and fifty cvcles of two hundred Hertz Hz signal and 351.75 cycles of 'two hundred and one Hertz tone. Since an even number of cycles has passed for te first tone, ItsPhase will be zero after the FFT is accomplished. However, the phase of the second tone Y61 be Mo hundred and seventy degrees from that of the first tone. Consequenfly, the difference between the phases of each tone is two hundred and seventy degrees which corresponds to an offset of.75, seconds between the clocks. Ift the OAT adjusts its clock by seconds, the one second 'tcks will be ailgned. In general, the phase difference defines the;me c-lisel. -FrIs offset is corrected in thiS implementation. 7-e mine correction process s represented by ste,p 1-08 in Figure 18 and s by the sof%Lvare in 'the OAT, as by --P)6, 1508'n t7e DAT block diagram c' Figure 21.

It shcuic 'Cle nicted. that the 'ones are generated in both the CAT and SAT in the same manner as the cr.ir.o signals were generated in the OAT. As described previously, in the preferred embodiment of the invention, a rrjic.rc,crccessor controlled digilka signal generator 1SW, 162-18 creates a pulse stream of any Irequency in +the band of interest. Subsequent to ceneraE[cn. the tones are converted into a three level sional at 1502, 1630 for transmission by the transducer 1200, 12,05 through the acoustic channel.

Alfter tone -=-cogniz,on and retransm ission, the OAT adjusts s clock, then switches to the Minimum Shitt Keying (NISK) modulation receiving mode. (Any rrodulaticn technique can be used, alti.uugh h Is preferred that MSK be used for 'the invention for the reasons discussed below.) Additionally, if 'the tones are prcper;y recognized by the SAT as being identical to the tones which were sent (stepp 1408), d transmits a MSK modulated command instructinci the OAT as to what baud rate the dcwnhcle unit should use to send its data 'to achieve the best bid energy to noise ralio at the SAT (step. 1410). ---Ine OAT is capable of selecting 2 to 40 baud in 2 baud increments for 'ts transmissions. 7ne communication link 'In the downhole direction is Mlaintainec a a;o baud rate, which rate could be increased if desired. Additionally,,he Initial message instructs the downhole transceiver of the proper ",-ansmissicn center frequency to use fcr,s ss transmi ions.

If, 'newever, ',G,,7es are not received by the downnicie ill reven tr, a,,aln. SAT did not receive the two lone transce ver, z wi -:1 1 acknowledgement sicnai sr-,ce CAT did not transmit 'hem. In 'his case the operator can either t,-,/ ser-c!rc,:f-,es however miany times he wants 'to or try recharacterizing essentially resynchronize the system.

In the case of sending. 'Llivo tones again, SAT will wad until the next tone transmit time during which, DAT would be listening for the 'tones.

If the downhcle receives the ton-es and retraps,-nts them, but the SAT does not,.',etec, them, the DAT will have switched to this MSK mode to await 'the MSK commands, and it w15 not be possible for a 'to detect the tenes which a,-.= tr;-:,,smltted a second time, d the operator decides to retrans.mit LJ'-ian,-, 1recharacterize, Therefore, the DATMI wait a set duration. Ift,'e NIS,< command is ncz received during Lhat period, 'ii switch back to ','e synchicniza, on mcce and begin sending ch' t w11 I L 111 1 Irp sequences every tNO Mini-tes. T-nis same recovery procedure wii] be implemented if the established communication link should subsequenCy deteriorate.

As previcusy menticned, the commands are modulated in an MSK format. MSK 'is a cl modulation which, in effect, is binary frequency shift keying (FSK) having continuous phase during the frequency shift occurrences. As r--.e,,:-nec abcve, the choice of MSK modulation 11cr use in the Sreferrec =-,m-;cdimenz oi the invention should not be construed as ir7,1tng ti,e inventien. For exam Pie, binary phase shift keying (BPSK), quadrature phase keying (OPSK), or any one of the many forms cf moduiation coud ce used in ti-,is accus,lc communication system.

In'Lne the commands are generated by the hcs, ccr7,cut-r 1'25 as,ords. Each command encoded by a cycillcai to prcvIde error detection and correction caiDablilt,. 7nus, 'Casic command is expanded by the addion of the error de'ec:;'cn bIts. 7-je encoded comr-nand is sent to the MSK modulatcr porlien of ',J'-e 68HC,l r-,jcr--,,oroces-z or's software. 7he encoded command bits control thle sarre irequency generator 1628 used for tone generazion 'c generae ti,Ie MSK modulated signals. In general, each encoderc command ciz is ma,c,ced, in this Implementation, onto a 115rst requency and tihe nexi is mapped to a second frequency. r--cr example, the channel center requency is two hundred and thirteen Hertz, the data may be m aPPed, -Inzc frequencies tvo hundred and eichtleen Hertz, representing a---V'and wo hundred and elcht Hertz, re.presening a The trans[,:ons me,;o frequencies are phase continucus.

U,ccn,,ec:ejving the baud rate command, the DAT wil send an acknowledgement 'c he SAT. if an acknowledgement Is not re-ceved by the SAT, it wlil resend he baud rate command if the operator deddes to retry. If an o,ce.,aor,,.;is,es, the SAT can be commanded to resynchronize and recharacterize he next set of chirps.

is sent by the SAT to Instruct the DAT to begin and ' sendinc da,---. lf an, acKnowledgement is not received, the operator can resend the command 1,1 desired. The SAT resets and awaits the chirp signals 11 the operator dec:des to resynchronize. However, if an acknowiedeement is sent I'rom tne DAT, data are automatically transmitted by the DAT crectly flollowing the ack,7owle,--4cement. Data are received by the SAT at se,s epreser-.,ed at 1J134.

Nom nally, the 'ransce'ver wiii transm. t for four r-,,,nu'es and,en s,co and lis.=n fer 'l-e nex, cc mimand frorn, Lhe SAT. Once the ccm.mane is rnce!vec, z'-,e DAT,.iii ',,ansmj anott-er 4 minute --,lcck of data. A'ernativc-!\/, the transMission cerlod can be programmed via the commands from 'he surface un' i' is icreseeable that,,he data may be collected 1'rom the sensors 1201 in the downhole Packace faster than they can be sent to the surface. There!'cre, as shown in Ficure 2 1, Itie DAT M, ay 1'nciude buffer memory 15 10 to store the incoming data from the sensors 1201 for a short duration prior '10 tra,-,,smirlir-ic it to the surflace.

7ne daza is enc--ded and MSK modulated in the DAT in the same manner thatthe commands were encoded and modulated in the SAT, except the DAT m, ay use a higher data rate: two to forty baud, for ti ansmission. 'I he CRC encoding Is accomplished by the microprocessor i 5 12 prijor to r,-, cdulating the signals using the same circuitry 1 55M used to generate tlie cnirp and tone bursts. 7ne MSK modulated signals are converled to tristate signas 1502 and transmitted via the transducer 1200.

the DAT and the SAT, the digitized data are processed by a c,uadraiu,-e cemedu.i-a'or. The sine and cosine waveforms generated by oscilators 1635. ',P-' are centered a' tine center frequency originally chosen durine the synchironization mode. initially, the phase of each osclilatcr is synchrcf-,i7-=-d to the Phase of:i'ie incom ing signal via carrier i hase of the incon-ing signal is transm, issi:cn. Eurinc recovery, tIhe P. 1 1 1 - 1 i -1 - 'rack' system such as a tracked ',c --,,/-,chrcnv via a phase inc Cosas icop or a oco.

Tr,e 1 anc G c-annels each use;"r,, te mpulse response 1 --- _. 1 -!i 1 1 U (F] R) low 'Cass;;".!iters -,E31S a resolonse a,D,GF--lxi;mateiv rnalches the bit rate. --cr the DAT. the filter res,L-icnse is fixed since the system always receives bit commands. Converseiv, the SAT receives data at varvine baud '7erefore, the fi'ers must be adaptive to match the current baud rate. Tne response Is chainced each tirne the baud rate is c,,anced.

Subsec,-,er,'ly, the 1/0 sam,,jiing aicorithm 1640 cptimally samples both 1 0 channels at 'Ll-.e aPlex of the demodulated bi-t.

However, optimal requires an active c!cck tracking circu, which is provided. Any c:" the many 'L,-ad't,cnai cock ',,--ckinc circuits would suffice.

a tau-dither c!cc'^" loo.c. a delay-lock tracking!cop, or the like. The cutpul of the 1/0 samPler is a stream of digital bits representative of the information.

The a.!'cn which was originally transmitted is recovered by decoding the bit stream. To this end, a decoder 1642 which matches -ter crocess. a CRC decoder, decodes and the encoder used in tne transmit detects errors in the received data. The decoded Inform, ation carrying data s used to instruc, the DJ-IT to acco m,,cish a new task, c 'instruct the SAT to - 3 receive a d ferent baud rp,,e, -c is stored as rece'ved 1 1 sensor data by the SAT's host computer.

The trans-.ucc-zr, as.7e in'er-,ace between the electronics and,he t,, ansm'ss on medium, is an mni cort-ant -seemeni of the current invention; 1 1 1 is Lherefore, ',,,,as discussed se.sara.e!y above. An identical transducer i used at eacin end of the link In this implementation, ait,c.-,gh it Is recognized ti-a n man.y si,ua!ons It may be desirable to use differently conficurec at ',-ie opposite ends of the comi-nunication link. In this imPiem entaticn, the system is assured when analyzing the channel that the link transrrl-er and receiver are reciprocal --no only the channel ancrinalies are anayzed. Moreover, to meet the envIrcnmental demands of the bore':-ioie, the ransducers must be extremely rugged or reliability is con-Promilsed.

---77HE MEASUREMENT-WHILE-DRILLING APPLICA.TIOW- In the foregoing desc,,lctlo,-i, the transducer and unication system are described as beinc used in a prod,,-,,ci.,ic Hov,,ever, Lhe transducer and co,-,,municaLion system car, aiso ce -.iiized, In a,,el;bcre during completion operations or drilling o,cera:cns. Ficure 2, shows one such utilization of the transducer and communication s,'s'em during drilling operations. As is shown, wellbore 601 extends f,,cm surface 603 to bottom hole 605.

Drillstring 607 is disposed 'L,,,ereln, a,-d 'is composed of a section of drill pipe 609 and a section cl' cinll coHar 6', ',. The drill collar 611 is located at the Icyiermost porlion of drilis',,'nc 507. and terminates at its lowermost end at rockb 613. As is conventional, during drilling operations, fluid is circulated downward throuch drl'lls',rinc 607 to cool and lubricate drillbit 613, and to wash formation cuttings u,c,,;ard ',nroug 1 n ---1-nu!us 615 of wellbore 601.

j Ypically, one c,, -,,,vo types of driilbits are utilized ior dr ll' I ing operations, Includinc (a) a rolling-cone type drilibit, which requires that driiis+tr:'r,ci 607 be rc,,a-,ed at sur,ace 6C3 to cause disintegration of the formation at Lo-Licm incie 605, and (b) a drag bit which includes cutters,,.hich are disposed in a;ixed position re!ative zo the bit, and which is retater., by rotation cl' drillstrinc 607 cc dy rotation o a por-tion cl driii collar ut lizat or-, of Motc11.

1 1 in eizhier event, a fluid cciumn exists within driiis,rinc &07, and a fluld column exists annuius 06 15 which Is betveen driiistrine 507 and wellbore 601. lt 'is ccr7,mon curing conventional drilling operations 'to utilize a measure ment -wh I le-d ri 1 line data transm iss.'ion system which impresses a series of either ccsive -or negative pressure pulses upon the fluid within annulus 615 to comr-nunicate data from drlil collar section 611 to surface 603. Typically, a r,-,easurerrenL-vh.lie-drilling data transmission system includes a piura!iry of Instruments flor measurinc drilling conditions, such as temperature and, pressure, and formation conditions such as formation resistiviry, formation camm, a ray disc,acce, and lormation dielectric ProPerties. It is convericr,,a to -,t!iize rreasurement-whlie-driilinci systerns c provide to ',1-icc-leraior a 'ti'-te sur.'ace information pertaining to tihe progress of the drilling operations as wei) as information per-,a' i ining to characterislics or qualizies of the formations which have been traversed by rcckbit 613.

in Ficure 24, m, easLrement-whlie-drilling subassernbly 617 includes sensors which detect Inform ailon perIaining to drilling operations and surrounding forr-nations, as well as tthe data processing and data transmission equipmen., necessar-y to coherently transmit data from drill collar 611 to surface 6C3.

A creat need exists in the drilling industry for additional information, and in paj-,cular n.['orma,,ion which can be characterized as near-,driIibit" iniormation. This is PanIcularly true for drilling configurations ,.hich utilize steering su 1 cassem 1 --ijes, such as steering subassembly 621, vinich allow for the crii'llnci of -,'rectior.al weils. The utilization of steerinc equiPment ensures thaz he rr, ea surem, ent-hie-drilling data gathering and transmission equipmenz is loca-ed thf'r-,v to sixty (30-60) feet from drill bit 613. Directional turns of drliibll 613 cannot be accurately monitored and controlled utilizing the sensing and data transmission equipment of measurement-while-driiiinci sys,-z.n, 617-, near drillbit information would be recuired in order to have a hic,-,er degree of control. Some examples of desirable near drillbit data include. in.-!lnation of the lowermost portion of the drilling subassembiy, the azim, uth of 'he lowermost portion of the drifling subassembly, driilbit tern-eraiure. mud rnotor or turbine rpm, natural gamma ray readings for freshly crilled 1,rm,,alions near the bit, resistivity readings for ",esnIv drilled 'ormations near he b,,. the.,e ch' on the bit, and the torque 1 1 1 1,!_ L on the bit.

In the Preent Invention, measurement subassembly 619 'IS located adjacent rockbit 613, and includes a plurality of conventional instruments for measuring near d,,lllb'(, data such as inclination, azimuth, brt temperature, turbine r.pm, gamma ray activity, formation resistivity, weight on bit, and torque or, bit, etc. This information may be digitized and multiplexed in a conventional fashion, and directed to acoustic transducer 623 which is located!7, an;-:-jacenL s-,bassembly for transmission to whi Is receiver 625, wh!cn, s loc-:ted the string, and ch adiacentri",easure,ment-,,vh,j;.--drilin,r,-s,ubasse-,,biy617. In this configuration, near-drilibit data may be ',,ans,-,,irted a shor', distance (typically th:rty to ninety feet) between,-an.smit',er 622 and reciever 625 which ilize the 1 ull 1 transducer of the Present as.;eii as,,le communication system of tjl,,ie present inven,,!cn, The ecr-,mun;cat!cr, svsTp-r-i cfl the present Invention continually monitors the annJus a c,,iaractert'zation signal lto'ldentLfy the optimum frequenc:es lorcommunication, as was discussed above. -Fne data may be routed from receiver 625 'c m easurement-while-drilling system 617 for storace, c,,ccessl,-,g, and reransmission to surface 603 utilizing ies.

conventional m easurem en,-,;,,iii--:,-drii;.inc data transmission techno g' 7nis provides an eccnomlcaj and robust data communication system for the dynamic and noisy environment adjacent drill collar section 611, which allows comr,-,.unicaicn of data for;ntegratien into a conventional data stream frem a,-,,easuremer,,-,ih'le-r-riiiine data communication system.

cr a,-ar-,sducer 627 may be Prcvided a surface 603 for,ecei,c c;, a,----us-.;c caa sienals from either one or both of transducer 623) or ',.ransducer 625. Or, aiternatively, and more likely, transducer 625 may bp utilized to transmit to an transducer located in the drlli,L,;ce section 6C9 of the drillstring 611 which will be able to transmit a greater distancethan transclucers;ocated in the drill collar section 611. In this manner, the transducers and communication system of the Present invention may be utilized as a data transmission system which Is parallel with a conventional measurer,-,en,-,vhiie-drilling data transmission system. This is useful, s!ri,-e ccnventional measurement-while- driffinG systems require the continuous flow of fluid downward through driPstring 607. During perlocs cf noncirculation or if circuaiion 'Is lost, conventional measurement-whie-cniling systems cannot communicate data from wellbcre 601 to surface 60", since no fluid is flowing. The transducer and commLnica'ion system c; L;,e present invention provide a redundant system which can be u.liiize,-- to transmit data to surace 603 during C.Uieseent pence 1 s wnen no fluld is'--eing Circulated within the welibcre. This provides considerable advanta:es since tnere are significant periods of time during which data comm, ur-,ica;on:s not possible during drilling operations utiilzinci conventional measurer-,en,, -,,hile-d, Aling technologies. In alternatve embodiments, the transcucer and communication system of the present Invention can be u'tl!ized to com, si'e,,eiy repiace a conventional measurementvhl'ie-drlilina data transmission s,,,,szem, and provide a sole mechanism for the communication of dista anc control systems within the we;lbcre during drilling operations.

THE GAS INFI-UX DETECTION APPLICATION- The transducer and communication system of the present Invention can also be utilized during drAling operaLicris,or tl-,e de.-c!c, of the undesirable Influx of hIch pressure gas into the annulus 01 ' a r,',s 'Is known to 'those skilled in the art, the introduction of high, Pressure cas into the fluid column of a wellbore during drilling operations can result in loss of control of the well, or even a lowout" in the rricst extreme situations. Considerable effort has been expended to provide safety ec,,,j,cment at the wellhead which can be utilized to prevent the total less of ccntrol of a well. Once a drilling operator has determined that an influx of cias is likely to have occurred, remedial actions can be taken to lessen the imoact of the gas influx. Such remedial actions inc!ude increasing or decreasine circulation within the well, or increasing the ---5 3 VISC13sit-y anc censi:y cf ",ne dri,ilirc fluld within the well. Finally, safety equipment can be utiizec 'to prevent total loss of control within a wellbore due to a sionifican'. ---:::s influx. The prior asi technology is entirely Inadequate in c)rovid!nc sufficient data to the operator during drilling operations vou-- C-:e C'Perator to avoid the many problems assoc:ated cas For-,L,,naiely, the transducer and communication svsten. cl the preser., invention can be utilized In drilling operations to pide the c-eratcr significar-t data pertaining to rovl 1 I,i 1 1 1 (1) whether an undesirable in,':ux --f cas nas occurred, and (2) the location of the gas "bubble" once 1. has enzered lohe drilling fluid column. It is important to note that an influx usually occurs as an introduction of a fluid slug, which is the gas In liquified form due ':) the hich pressure exerted by the fluid column.

Since the cas 7as a c,,er density, i1 will rise within the fluid column; as it nses, it -,,me out cl; solution, and take 'the form of a gas bubble".

In acc-jrc,:-::nce w'[.h, the c'resent invention, an influx of gas can be detected in a fjuic column within a weiibore which defines a communica,lcr,. cy the llollo,,ng steps, (1) one actuator is provided in communication with the wellbore for convers!or,, of at least one of (a) a provided coded electrical signal to a corres.ccnc ' jnc generated coded acoustic signal during a message transmission mode of oPeration, and (b) a provided coded acoustic signal cc a cenerated coded electrical signal during a message recePtion mode of operation; preferably, only one actuator/transducer is ErovIded, and this Is located at the surface of the wellbore at the vel!hea(--, End is in 'i'L,,!d communication with the fluid column iments cne vithin the annulus cl although in alternative embod or more transducers may be Provided clownhole within the drlils',ring-, (2) the transducer s utilized to generate an in'erregatng sicinal a: a seiectec ccation within the welibcre', the characterizing signal may be a "chirp" v,icr, nc!udes a plurality of signa! cOmPcnents, each navino a dli'ieteni:;,,ecuer-,cy, and sPanning over a preseected range c,' frequencies, cr j: r-,,ay be an accLszlc signal which incluces oniv a single requency corri,ooner7z., (3) ne transducer Is uzilizedto aPplythe interrogating sicnal Lo the communication channel which defined, preferably, the liiuid column within the annulus; Inzerrocating signal is transmi"ed through 'the communication channel and is receNed by either a Cii'c-rent transducer, or s echoed back uPward throuch the communication channel and recerved by zhe transm i-LLIng transducer; 7,e)(,,, 'ohe interrocating signal is ai-,aiyzed 'c Identity at least one nf the foilowing: (a) portions of a preseiected range of llz,-equencies, vvhlch are suitable for communicating data in the wellbore. these portions may be identified by either lrequency or bandwidth or both, or by slenal-to-noise characteristics such as a signal-to-noise ratio, or signal amiblitlude; (b) comrrinicajon channel artributes, such as C, 3mmunic.,:tjon channel length, or communication channel Impedance; (c) signal artribuies, such as signal amplitude, signal phase, and the occurrence of ioss G't the signai., (6) Finally, the steps of utilizing, applying. receiving, and analyzing are repeated periodically to identify changes in at least one of (a) porlions of the preselected rance of frequencies which are suitable for communicatIne data 'In the veibere Including frec.uency changes, bandwidth changes, chances in a sienal-lo-ncise characteristic, chances in signal amplitude of s[Q,7a!s trans,-,iitLed within the por-len, and signal tirne delays for sionals transmitted withiln the portion, (b) communication channel 'nclud'no chances in communi - anri 1 1 c,,ticn channel length or communication channel imPedance, or (c) changes in signal attributes (elther interrogazinc signais or subsequent signals) including changes in signal amplitude. chances 'n signal phase, loss of signal, or signal time delay.

When a s'lr.,(-,!e transducer Is Llilized, in the preferred embodiment of the present 'invention, such transducer should be located at the sunace, and snculd be ul:lized to transmit a signal downward within the communication channel (of the annulus). Typically, the acoustic signal is reflected off cl the drill c--lar por-tion of the drillstring, and thus travels back upward through the commun [cation channe[ 'it 'is received by the transducer which cenerated:he signal. In fact, any signal provided by the surface transducer wii; ',;avel a multiple number of limes downward and then upward within the communcation channel as the signal repeatedly reflects off of the drill collar portion of the drillstring. In one embodiment of the present invention, cne or more acoustic markers may be placed within the drillstring at selected locations. Each member is generally larger in diameter than the adjoining drillstrinc, and provides a reflection surface at one or more known distances. The reflection of acoustic signals off of these markers is monitored for c'-lances which Indicate Its presence of gas.

Ficure 25 C:rapn,Ca'lly depicts a laboratory test of the transducer of the P,,;--,--sen, invention in a wellbore five hundred (5W) feet deep. In this figure, the X-axis is representative cl; the acoustic travel path in units of time, which have been normalized to units of length, and the Yaxis is representative ef swai strength of the signal received by the transducer vhch is c!s-.osec a,,,e sur-i'ace. Peak 701 is representative of a signal which is ger-,era-,-.c by z,7e sur-face accustc transcerver. At the termination oitime Interval 7'C'I, '1-e first echo 705 is detected by the surface acoustic k,,ansce!ver. Dunno this time 'Interval, the acoustic signal has 1-aveled downward throuc- the annulus, reflected from the drill collar, and ion. At craveled back upwarc to tl'-e sunace acoustic transceiver for recepti the termination oil tim e inerval 707, the second acoustic signal 709 is received by the sunacee acoustic transceiver. At 'the termination of time intervai 711, the third -c-nus,ic echo 713 is received by the surface acOustic transceiver. At tne termna,,ion of time interval 715, the four-th awustic echo 717 is received by the sur-,a--e acoustic transceiver. At the termination of time Interval 717, the fifth echo 719 is received by the surface ac- oustic:

transceiver. At the term, ina-Lion cl; 'Llme Interval 721, the sixlh echo 723 is detected bv sur,'ace accustic trar-,sc--!v,,:r. At the termination of time Interval 725 seven7 ec-lc 72-17 is detectec: 1,;y the surface acoustic transceiver.

Thus, it Can --e seen tnat If the annulus is unobstructed, a regular pattern of echoes can be expected for acoustic signals emeted by the surface acoustic tra,-,sc,..ver. Each echo occurs at a predetermined time on a time line, which -,cr,,espcn.ds to the distance between the surface acoustic transceiver and ','e drill coilar portion of the drilisiring. Since the length of the is.7cvn, and the frequency of transmission of the -j7 acoustic signa! is also known, '.'e ec7oes occur as expected, unless an obstruction exis-,s wit',r:n the annulus cf, the.,,eilbore.

An inllux of cas in,,o the annuius can serve as an cibstrucion which will cause the occ,-,.,rence cl echoes to be shifted in time. This occurs, since the gas "slue' or "bubble" has r-li-;ierent acoustic transmission Properties from, the d,,!iilrc: mud. and wlil cm-vide a boundary from which reflection is expected. Thus, z,-,e cenera-lon of an acoustic signal by the surllace acoustic 'ransceiver, and sj'-Isecu-ni monitoring of the return echoes, can be to detect (1) 'l-le presence of a gas influx, and (2) the location of a cas Influx. Assu me for example that a gas bubble has entered the annulus durine drillinci c,cerazicrs, and is located at a position m'dway bel-ween 'he sunace acoustic transce ver and the drill collar. The I -i expected result is an ec7,o whicr, indicates a travel path of approximately one-half of that was Previously encountered during monitoring. The operator at til-e surace car., analyze the echo pattern and thus deternnine i,e presence and ioca-,lon cf the gas bubble.

In add"'on to r-oni,tcrir,.c the encith of the communication channel, the transducer and cor-,,municaL1'01-i system of the present Invention may be utilized to detect the influx of gas by monitoring the extent of amplitude aCenuation In he echo s'lgnals as compared to amplitude attenuation durinc periods of oPeration duninc which no gas influx is present within the communication channel., said m onitoring is preferably not a calibrated measul ement but is instead a reiaL!ve comparison of attenuat.ion and the desc,,iptcn which follows utilizes the term "ampidude attenuation' in this sense. With reference aciain to Figure 25, Lhe presence of undesirable cias bub".-,,'es t,"e column comprises a communication -- g,-e - channel will result in a chancie n acoustic mn, pedance of the fluld column and result in additional reflecl,!on losses. This change in acoustic impedance of the fluid column, n result in a change in the amplitude attenuation of the sicinal as It ecnces witnin the wellbore by traveling downward and upward. For exam, Pie, 1f a larce amount of gas is present within the communication channel, a greale, or lesser degree of signal arenuatic)n mav be obser-ved tlhan is norm, ally encounlered during periods of operation dunne which no cas is present within the communication c,annel. Therefore, by continuously monitoring and comparing a-,Lenuation values, the transducer of the present invention can be utilized to detecl, chances in acoustic impedance which occur due to the influx of gas within the communication channel. Any detected change in communication channel length or impedance can be ccnsIdered to be detection of changes:n 'communication channel a-tLributes".

Slenals which are transmitted from the transducer can be monitored for chances in amplitude, or significant time delays, both of which could indicate the presence ot an undesi.ralple gas influx. Additionally, s; cinals which have ceen tra,-sm-,,ed bythe transducer can be monitored for signal phase si-lift, which in an acoustic transmission environment corresponds to significant ransmission delays (which are fqr greater than one wavelength).

The transducer and communication system of the present invention may also be utilized during a cas influx detection mode of operation, wherein the process of selection of the one or more portions of available bandwidt,, for data communication 'Is utilized to detect changes in ',le com munlcat:on channe! which indicate,,-iat a gas influx has occurred.

As is shovin in Ficure 26, s.-jrace acct-,sic transceiver 743 may be coupled in a position at the surace to communicate with annulus fluid 741 within wellbore 735. Drilling rig 731 is p,,c,jded to rotate drillstring 7753. As Is conventional, drillstring 733 includes an upper section of drill pipe 737 and a twer section of,,rill ccilar 739. Rockbit 738 disintegrates geologic icorr-,ia'!ons as drillstrinc 7-"--,:s rc,.-:::-=,- relative to welibere 735.

Dulinc selec:ed ccr;cns cl' the dIniling operations, sun 1 ace acoustic transceiver 7413 (and assce:a,,-.d Personai computer monitor 745) ;'zed to transrn't:cnals downward into wellbcre 73 :s ui!,1 t,rcugh annulus fluld 71i,,hich is,,,,e communication channel. One or mi ore reflection markers may be Provided and coupled in posi-ticin within drill pipe section 737 ci drlils'Lrlnc 73-1 A;'e,,nati'vely, the reflective boundary provided by drill collar 739 mc-y be utilized as a refiection surflace. Suri'ac-acoustic transceiver 743 ',rans,-,,its either (a) a signal which includes a number of signal components, each. having a different frequency, spanning a preselected frequency range, cr (1s) 'transmits a signal having a lrixed d.cwnward through annulus fluid 741, irequency. The sional 'is ore,caca,;=r- 1 and reflect,-, Off of drill c--;'ar 739, a:r-d returns toward the surace; 1cr recection 1 cv sjrTacrl- accus"Lic 'transce',jer 743.

If a signal 'is transmined which Includes a number of different frequency components, the suriace acoustic transceiver can anaiyze the sicnal-loncise attributes oi'various frequ-ency por-,icns over the preselected frequency range to identity one or rn, ore optimal bands wi-thin the frequency range, t-ypicaily each being approx!rnately ten (10) Hertz wide, which are opILIrr,a! at that time for the commLr-,!ca,ion of data within weilbore 735. T-he oa,-tic,-,iar bands rnay be Ide.7'ii'ied by Lcper and lower frequencies, or a center frequency ann,' a In either characterization, a spec:"lc portion or a requency range s id 1 entified as being preferable to other por-tions cl' the frequency rance j'cr tll-,e efflicient transmission of data.

-ibe introduction cl' an 1-.ndesirable cias Inilux into the annulus '1ul'd 741 vz,,n'n weilbore 735 wii]

J acoustic Impedance of e annuiLis 'luid 741, and thus wfil aiter clstim al frequency portions for da-_a zransm, Ission. Data can be by continually characterizing the corn, m, unIcalion channel cf 7 I during periods in which no gas influx is present,ithin annulus 74 1. Subsequent characterizations cf annulus fluld 741 can be corn,oa,-ec to the hisiorical data to identify changes In the optimal bandPass pc7L.ons cf the preselected frequency range 'to identify 'the occurrence of a infi;j.-,x.

In Figure 26,,ocKbit 738 is depicted as traversing a high pressure gas zone 7-47. This causeS a gas 'influx 749 to enter annulus fluid 741. Typically, gas influx 7,'--c en,,el, annulus fluld 741 as a "slug' offluid. As it rises, It wil come out cf sc';u,:cn and become a gas 'bubble". T-he presence c.i'elzjlier the fluid sluc cr,-,e cas bubbile should cause a significant cnance in the optimal oPeratinc i, ecuencles for ihe communication channel of annuius fluid 741. 7nese atDrL,,c,L cnanges in the optimal data 'transmission frequencies should provide an incicarion to the operator at the surilace that an undesirable gas influx has occ-rred.

In alternative er-,ccci,-,,ents, one or more transd. ucers may 1 be located within drillstring 733 fici- 'L7e 'transmission and/or reception of acoustic signals. For exarrcIle, acoustic transceiver 740 may be provided in a position adiac-en't --zar 739 for the receipt or transmission of acoustic signals. In this c--n,ic-uratior,, downhoie acoustic transceiver 740 may be utilized, as was described above in connection with the description of the data communication sysiem, to generate a characterizing signal which is detected by s.urface acoustic tiransceiver 741, and processed by PC monitor 745, also as was cescr:,.ee above. Sunace acoustic transceiver 743 and clownhole acoustic transceiver 740 may be utilized to transma sicnals back and lor"Lh, across tnecGrrmunicat'!on channel of annulus fluid 741. Changes in the comm.unication channel, chances in signals liransmiked be,een sunace accjslc transceiver 741 and downhoie acoustic transceiver 740, as vel as c,,ances in the optimal communication frequencies can be utilized to detect ',he entry of an undesirable gas influx 749. Echoes which are generated within the communication channel of annulus fluld 741 which rom. e!zher the surface acoustictransceiver 743 or the downhole acoustic:ransceiver 740 can be utilized to pinpoint th-e location and size of a gas bubbe as rt travels upward within 'the annulus of the wellbore.

TIne preseni inven;on can be utilized to monitor gas influx into a well durinc drillinc, and' de7ec, -,,-.e evert prior to the influx bubble reaching the surface. This wiii oreativ r-,.crcve safety, by preventing blowout of the well or other serious loss of cOntiol stuatIons. -Fine system can be utilized to detect the position of the LoP, of the bubble. Since the transducer and communication system cl; th:e present invention does not require that crcUlation be present within ',-,e weilbore, the present Invention can be utilized to detect the influx of cas curing quiescent periods during which no fluid is being circulated within the wellbore, such as tripping and casing operations. The p resent also allows for the detection of small gas bubbles, far e-nrler than is undeir conventional techniques. The Qresent also alc,..is fcr sgnif"cant changes to occur n the,.ell during drilling operations. s,,-,cn as changes in mud weight, and he subtraction or addition oi driiis,,-ing sections, since the system allows for continuous monitor[nc of 1,11-,e ccrrr-,,unication channel to determine optimum operating frequencies. TnIs feanire allows for the automatic and continuous adjustment of the "baselire" ance during significant reconfigurations of the wellbore, withcut requinno any significant knowledge by the operator of acoustic syster-ns. In snon, aiered acoustic paths, disrupted ac- oust!c returns, disrupted frec:.uerc., channels, and changes in the tirne of Fight as well as changes in ar-,,,cij"ude reiative to previous amplitudes can be utilized separately or together to centi. the occurrence of an undesirable gas influx, and once the influx has been detected, can be utilized to pinpoint the location, and perha,r-,s size. cf cas influx.

ALTERNATIVE DATA COMMUNICATION SYSTEM. As an al-ternative to identifying specific and nar,-o,,,i Ponlons of a frequency band which provide optimal data t,-ans,-7,issic-,, 'e communication system of the present invention can utilize an o,c,cosi-Le approach which utilizes a very broad band in its entirety lo transr-,!iL a CC),,reScr.,r-,dlng binary character, such as a binary one, and which uses anct-,er croad band to identity a corresponding binary character, such as a b'inar-V zero. It has been shown by Drumheller, in an article entitled "Acoustical Properlies of Drilistrings", Sandia National Laboratories, Paper No. SAND88-0-502, published in August of 19B8, that acoustical signals of specific frequencies travel from the bottorn cf a drlilstring to the sur-face with cnly s miall atienuation. These frequencies are contained within frequenc, 11, bands. Within these frequency bands there c-an be wide variation of 'the a-enua,,on of any one particular frequency, bLI some or most of the the band pass through the drilistring now;thstandine dramatic changes in the welibore environment. Thus, selecting one particular frequency band as the modulation frequency for a data transmission system ensures that there is only a small probability that P-11 frequencies within the band wiii be attenuated and lost.

In accordance with the present Invention, the communication channel is In the we!lbore, either a fluid ccumn or a tubular rnember, is analyzed to determ ine an o.cijmal frequency band which may be iPIiized to designate a paricular binary value, such as a binary "one", while another separate frequency band is identified to represent the opposite binary character, such as a binary "zero". For example, the communication channel Is Investigated to identify a broad frequency band, such as five hundred ninety Hertz to six hundred and ninety Hertz (555-90-6,190) which corresponds to a binary "one", while it aiso Investigated for a separate frequency band, such as eight hundred and twenty Hertz to nine hundred and twenty Hertz (820-920) which corresponds to a binary "zero..

The transducers of the present invention are Tillized to cenerate an acousical sional,vnIch includes a plurality ofi signal porlions, each portion representing a different frequency within +the band, the portions altogether spanning the entire width oil the selected frequency band. For example, for the binary one, the acoustic transducer will produce a signal which includes a piurality of sionai components spread across the five hundred ninety to six hundred ninety (590-690) band,it'dlth. LkeWse, for the binary "zero", the transducer will generate an acoustical signal which includes a plurality of signal components which span the range of frequencies between eichthundred and twentv Herz and nine hundred and %Lventy Her-z (820-92,'-',).

L.L - During a reception r-,-.cce of oPeratIon, the transducer, and associated microprocessor comicul-er, is utilized to analyze the energy levels cl' acoustic signals detected in ',le separate frequency band ranges.

Preferably, the enercy cif the --ero tand is compared to a baseline noise !e,ei which has pre,.,icusiy bee,-7 n-c.ai,ee, cr the range of frequencies.

Likewise, 'the enercy '-=xje! cl; t,-e range representative of the --zero" is a -ase; ne enercy level prev' tinary icusily acquired cr the same f,-ecuene., -ance.

These c-,,-ice,ots are n block diagram forr-,i in Ficures 27 and 28, with Figure 27 de,cicll!rc 'tj-;e logic associated with the transm-Lter, and Figure 28 deplc.:'r-.c the locic associated with 'the receiver.

Referrinc, first to F':clure 27, sensor data is provided by sensors 801 to microProcessor 805 and C:tItal storage memory 80-1. When Lransm ss on of the data is desirec. 805 actuates digital-to analog conver-ter ceneraes an acLation signal for binary 'ones', and an actuation sicnai for b:nan. '7. rces". Power driver 809 generates a unique power sicnal 2ssoc':a'e,i "-.)i,,ary zero, and a unique power signal associated with each binary cine, as Is depicted in graph 811, with a iirstt preselected rance of frequencies representing a binary "one', and a second preselected rance ol freque-ncies representing a binary 'zero". In the example of Ficure 27, 'rec.u-rc;ecz n C-e range of five hundred ninety to S;x hundred and ni'ney Herz ar-e representative of the binary one", while frequenc:es in the,2nce cl e'!ci-t hundred and twenty to nine hundred and twentv Hertz (820-920) are rePresentative of the binary 'zero.

This driving signal is suppliec o.,a-,scucer 813 which is acoustically cou,ced 'c tl',e c,,-,,-rmunication channel, which is preferably, but not necessarily, a,'il-,jd column within the wellbore.

7,e acoustlic signal is conducted ',c a rernotely located ansceiier, as ransducer a', S cf Ficure 28. 7ne received acoustic s!gnals are at amplifier 817, and suipplied simultaneously to bandloass i'iitp-r 7--'G and band,Pass 829. n the example c'[ Ficures 27 and 2, ite, 819 Is a bandizas-s allows for the Passage of irequer-,c,:es!,- -,ie r-:r-,ce cl; five hundred nine-.y to six hundred and ninety (590-'--C) Her-,z. band.pass filter 829 allows for the passage of frequencies in rance of eicht hundred and 'L,,,;en'ty Henw to nine hundred and,Yen-ty Her= (820-920). The outputs cl; LbandPass -=rs 819, 829 are suppiled to signal processi:nc Ciocks.

Xvicre specificafly, the cut.cut cl band.pass fifter 819 is supplied to Intecrator 821,,hic.h Provides as an output an indication of the energy content of tne S;cnals in the rance cl: iiec.Lenc:es corresponding to the binary "one". 'the output: of band.pass fiflter 829 is supplied to inl,ecr,-m,sr 831 Pr-'vIdes as an out3Lt. _an indication of 'the energy contained by swals in the rance or frec,,,enc:es corresponding to the binary "zero". E-Sa:se band integrator 823 is utilized 'c provide an indication of the energy eve! contained within the rance of frequencies corresponding 1 - 1 1 1 1 is present.

to the b'nar,; d,,r'nc oer cds wh'cn no s cral Lkewise, base band 833 is utilized to PrcvIde as an ot-itput an indication of the enercv c-.n-::-e,--,,,ithin the frecuencv band =responding to the binary zero" durinc --:ods of inactiviry. As 's shown Figure 28, 'he ou-tput of integrator 82-, ase band megrator 823 is supplied to summing ar7,Diiier 82-15. Lkewise, t,'e cl ln.p-gratGr 831 and base band ntec,ral,or 833 are sucp(led c mr-,,ine am, oililer 83-11 Tne am, Clifiers 825, 835 are prov ided to a ccr-,,.caracr.,:' the cl' 1525 exceeds ',e ci summIng ampil-lier 83 5 c--,-,,caratcr 827 Is a --,i'nar-li "--ne".

r,c,,,ever, If 7e Gui= ct greazer ',7an ",ne cl; sur-,,m r-,,c cl" 827:s a -,nar- !n linis a'a 3rc,icec:, as an r c M 7,1C,,cprccesscr 8G5 a e c u z c; in a 2,27 ------ Of course, in the present invention, the transducer which 1-s described herein may be utilized as an acoustic signal generator. Furthermore, the data communication system described herein may be utilized to select the best range of frequencies 1Z tor representing the binary "one" and the binary..zero".

67

Claims (10)

CLAIMS:

1. In borehole communication, a method of preparing for communicating data between two locations, at least one of which is within a borehole, using travel of acoustic waves in a transmission medium which extends at least in part into a downhole location, comprising:

(a) generating a first synchronizing signal at a first location; (b) acoustically transmitting said first synchronizing signal through said transmission medium to a second location; (C) receiving said synchronizing signal at said second location; (d) synchronizing said signal between said first and second locations; (e) generating a second synchronizing signal at said second location; (f) acoustically transmitting said synchronizing signal through said transmission medium to said first location; (9) receiving said synchronizing signal at said first location; and (h) synchronizing said signal between said first and second locations.

68

2. A method according to Claim 1, further comprising:

(i) after synchronization is complete, sending data between said first and second locations.

3. A method according to Claim 1, further comprising:

(i) after synchronization is complete, bidirectionally transferring data between said first and second locations.

4. A method according to Claim 1, 2 or 3, further comprising:

generating, applying, receiving, and analyzing a characterizing signal in said transmission medium to identify portions of a range of signal attributes which are suitable for communicating data between said first and second locations.

5. A method according to Claim 4, further comprising: sending data between said first and second locations utilizing said portions of said range of signal attributes; (k) continuously generating, applying, receiving, and analyzing a characterizing signal during said step of sending data to 69 identify portions of said range ofsignal attributes which are suitable for communicating data between said first and second locations; and (1) switching between selected portions of said range of signal attributes to optimize communication of data between said first and second locations.

6. A method according to Claim 4 or 5, wherein said characterizing signal is utilized in synchronizing said signal between said first and second locations.

7. A method according to any one of claims 1 to 6, wherein said steps of synchronizing include:

calculating at least one clock error; and correcting said at least one clock error.

8. A method according to Claim 4, 5 or 6, wherein at least one of said steps of synchronization includes utilizing a center frequency selected through analysis of said characterizing signal to synchronize said first and second locations.

9. In borehole communication, a method of communicating data between two locations using acoustic waves in at least one transmission medium extending at least in part in said borehole, including the steps of:

(a) periodically characterizing a dynamically varying number of acoustic channels with dynamically variable channel attributes in said at least one transmission medium to determine a preferred transmission frequency for communication of said data; (b) synchronizing a first communication device and a second communication device; and (C) bi-directionally transferring data between said first and second communication devices.

10. A method according to Claim 9, wherein said step of synchronizing comprises:

synchronizing said first communication device with said second communication device; and synchronizing said second communication device with said first communication device.